The account from the Old Book of Tang listed on the Wikipedia page for this lists 20-30,000 people being eaten due to the defenders of Suiyang starving, which seems difficult to believe or fathom. I know starvation is a common factor in sieges, and sometimes cannibalism can occur in desperate circumstances but is cannibalism on such a scale accepted by historians studying this rebellion?
There will be more to say about the specifics of your query, but in dealing with an earlier question about the supposed "massacre of Guangzhou" (878) I discussed the Old Book of Tang as a source and made some comments on how to read the sorts of numbers you are citing here. You might like to review that answer while you wait for fresh responses to your query.