The main issue with this is Twitter's responsibilities going forward, not that Trump was fact checked (because he lies about lots of things that should be pointed out). As you said, this puts them in a legally grey area and might bite them in the ass down the road. However, I don't think Trump can shut them down for "censoring" him as 1. he wasn't, and 2. he is definitely lying. But, it SHOULD be the US government telling him to stop (they won't) or people suing him for lying to the public and getting a court to force him to stop (since he is a public figure but not sure if there's legal standing there).