I seriously hate it whenever exchanges plan to *potentionally* reverse trades if things don't go according to their plan. If things are about to get serious, and they don't think they will have everything in full control, then just halt trading temporarily. Reversing trades is a shameful practice. That being said, I like how Huobi explicitly states that people will be given full access to every alt as result of a chain split. I think every exchange should be doing this, but obviously, the motives of the exchanges not doing so are clear. And as expected, BIP91 signalling has gone over the 80% level, but not yet (according to coin.dance/blocks) in a stable manner to keep it above 80%. Give it a day or two, and it won't drop below 80% anymore.
It's up to the exchange to rollback trades. It's up to you to choose which exchange to trade.