While there is always more to say, I addressed the problem of the Guangzhou massacre (and the question of what we can actually know about it; the sources are very problematic and the death toll you cite is extremely unverified) in an earlier response here. You might like to review that thread while waiting for fresh responses to your query.

How did the sack of Guangzhou in 879 affect the city and the transoceanic trade with the Abbasid caliphate that terminated there?

Then, see a second thread also relevant to your question, which attempts to assess the thorny problem of the number of black people potentially around in Tang China:

During the 9th century, a Tang Dynasty author wrote a story about a Black Person ("Negrito") active during the 8th century in the Tang Dynasty. How many Black People, Negrito or otherwise, were in Tang China, how did they get there, and what were their lives like?

Both with me, u/mikedash