Previously there was still hope of selling TikTok globally.
More importantly it seems that Kevin/TikTok has no influence at all in the negotiations - it is purely between Bytedance and the relevant companies. This is probably the real reason he's leaving - he's in no position to shape the deal and it isn't looking good for him right now.
If there wasn't a strong leader already to take his place then it would be a very negative decision on Kevin's part.
That said, surely his contract has provisions for a payout should his company gets acquired. Wouldn't leaving before any transaction close void that?
http://en.people.cn/n3/2020/0815/c90000-9721956.html
https://www.thesun.co.uk/news/12295304/boris-johnson-clashes...
I am guessing it will be official soon.