Twitter has rules (so does Facebook, as anyone who has run afoul of them, not moi, will attest) that Trump has repeatedly violated.
The company refrained from holding him accountable.
The fact check was with regard to their new policy to block attempts to suppress voting in the 2020 election.
For example, his egregious slander of Joe Scarborough (and the pain inflicted on the woman's widower) should never have been permitted under Twitter's rules. I can't remember any President, and practically any politician at all (except for a handful of fringe people) who have stooped that low - the GO is not just unfit to be POTUS, he's unfit for any decent society. You have to go back to Joe McCarthy to find a parallel at the national level.