The names of real places in east Africa as given by Chinese authors are a bid different.

昆仑层期国 or 崑崙層期國 (Kunlun ceng qi guo) Madagascar; Li Kung-Lin (d1106)

层拨国 or 層撥國 (Ceng Bo Guo) Zanzibar; Chao ju kua (1226)

桑骨人 or 桑骨八 Sang gu ren (Zangi people) or San gu ba (Zanzibar) Chu Ssu-Pen (1320)

桑骨人奴 Sang gu ren nu; Zan-gi people slaves. Chu Ssu-Pen (1320)

I however think they were from somewhere in South-East Asia.

Sui shu (History of the Sui dynasty for year 607)
Yuan 82 Biographies 47
(about the kingdom of Chitu, in the Malay peninsula)
The king has three wives and the kingdom embraced Buddhism. (The king) lives in the city of Sengzhi    僧祗城 with a series of three gates about 100 bu (steps) from each other.

Note: the writing Sengqi (僧祇‎) is the same as for the slaves. And also on the Malay peninsula was the place where according to G. Schlegel: Ho-ling the country who donated the slaves to China was located.

Jia Dan (801)

To sail out of the west [mouth] of the Strait, for three days, one can reach Gege-sengzhi-guo 葛葛僧祇國[Kakap Sengi] Kingdom;(1) this is an island located to the northwestern (border) corner of Fo-shi Kingdom; many people of this kingdom would violently rob other people, so voyagers feel fearful.

(1) is an island of the Brouwers Archipelago beyond the northern coast of Sumatra. (Schafer’s Kat-kat Zangi country). Here the writing Sengqi (僧祇‎) is the same as for the slaves.

Lingwai daida by Chou Ch’u-fei (1178)
The most important festival of the country (Dai Viet) is on the 15th day of the seventh lunar month, when every family exchanges presents with others. The officials present Shengkou (Zeng-ki)(slaves) to the king, who will give the officials a feast on the 16th day in return.

Note: shengkou (生口), initially meant 'captives in war' but then also meant 'slaves' in the Eastern Han dynasty. So it might well be different from the Sengqi (僧祇‎) slaves.

From the Arab-Persian side there were also several places in Indonesia that had similar names.

-Masudi (± 915) talks about the sea of China or Sandji, which is the furthest of all. G. P. Rouffaer identifies this with the sea of Djenggi or Papoea-sea from the Javanese history. Other authors however say that what is meant here is the Sea of Japan.

- Ibn Majid’s navigational tract from 1462 mentions Salat Zanji (the Zanji Strait) close to the Karimun islands.

Several more instances of words being transliterated as Sengki or Zangi  in the same geographical area are to be found on my webpage: Note on Inscriptions in Java . There is little more then the phonetic resemblance of the words with the persian-arab word Zanj and the Kawi word Jenggi and the fact that those people are slaves to support the statement that these people are African; this is just not enough.

Ge Chengyong 葛承雍. 2006 (see under) considered Sengzhi an expression of Buddhism (seng=monk) in Nanhai in early times. This idea has some merit. Sengshi is also the name of the Canon for the Buddhist Monks; from there that sengzhi can also be translated as the ‘Monks-Wisdom’.  This makes that: the donation of a sengqi-women (a nun as seng= monk) becomes the donation of a literate virgin. And the donation to the emperor of sengqi-slaves; becomes the donation of educated slaves. That would be really fitting as present for the emperor. 

Ge Chengyong 葛承雍. 2006. "Tang chang'an heiren laiyuan xunzong" 唐長安黑人 來源尋踪 [The origin of black people in Chang'an City of the Tang dynasty]. Zhonghua wenshi luncong 中華文史論叢 65: 1–27.

Zhang, Xinglang (1930) in his article: Importation of Negro Slaves to China Under the T'ang Dynasty asserted that the Negro slaves were imported by Muslims from Africa into China.

In 2001, Chinese historian Ge Chengyong criticized Zhang’s view in an article to probe the origin of Black people living in Chang-an city in the Tang Dynasty and drew a conclusion different from Zhang’s. Considering the thesis of the “African origin” not convincing enough, he suggested that the Black people were not Negroid from Africa but Negrito from Nan Hai (south sea, present Southeast Asia and called Kunlun). Some of the Kunlun were part of foreigners’ annual tribute to Chinese authorities, some were left in China by foreign envoys, and some were enslaved people sold to the coastal regions. As for the term Sengzhi, it is generally considered to be identical to Zanj (other forms such as Zinj, Zenj, Zandj, Zanghi) and is a word used by the Arabs to refer to the East African coast. …… Ge did not agree with the view, considering Sengzhi an expression of Buddhism (seng=monk) in Nanhai in early times. It is more proper to look for the origins of Black people in Southeast Asia rather than in Africa (Ge, 2001), a view that is accepted by scholars in China (e.g. Cheng, 2002; Liang, 2004)

Ge Chengyong: Some of my peers and predecessors have thought that these dark-skinned people, with curly hair, broad nose and thick lips, hailed from Africa. I believe they are more likely to have come from Southeast Asia, for example the Indonesian archipelago.

Between the fourth and sixth centuries, to which the pottery figurine (of black people=Kunlun found in China) has been traced, the trade route between China and Africa merely connected the Chinese empire with Egypt. So there is a very slim possibility that these men came from the heart of the African continent. On the other hand, words about them, albeit scant, appeared in writings of the time in which they are portrayed as 'wearing shorts and being superb divers or fast mast-climbers'. These are the traits associated with islanders from Southeast Asia."

Some of these men may have been purchased - or captured - by local tribal leaders or human traders in one of those Indian Ocean islands. From there they could have traveled to what today is Vietnam before moving further toward the heartland of the Chinese empire. Their existence in Tang China has cast a tantalizing beam of light on the society of their adopted home, although most details of that existence are likely to remain forever shrouded in mystery.

Early Global Interconnectivity across the Indian Ocean Angela Schottenhammer · 2019

(About the Senqi slaves brought to China)

We confront the fundamental question of whether these individuals with origins unspecified by the sources were in fact even actually African. At least initially, having only exceedingly rare and incidental contact with them, the Chinese applied the terms sengzhi and cengqi not to Africans at all but instead to the welter of Indonesian archipelago tribal peoples with whom they had for centuries much more commonly and frequently interacted. We can certainly assume such to have been the case from the inception of the use of such terms through the duration of the Tang dynasty. Indeed, entertaining the prospect that neither the enslaved children from Java nor the enslaved maiden from Srivijaya were necessarily African, the late sinologist Edward Schafer (1913-1991) speculated that these human offerings to the Chinese court at Chang’an may well have been of Malay-Negrito stock. Indeed, even as early as Tang times, the Chinese were evidently aware of an island northwest of Eastern Sumatra or Palembang (=Foshiguo) that was called Gegesengqiguo (Schafer’s Kat-kat Zangi Country), whose inhabitants are described as: prone to violence and who, once they have boarded their boats, are much to be feared.