It's within their rights to do take these actions, fact checks and hiding/deleting tweets, to protect their ecosystem. If it is questionably legal because it may influence the election, then I haven't seen the law it is breaking. I see a better argument for showing Twitter promoting Trump's feed to drive clicks as an in kind donation which could quickly break legal campaign donation limits.
Twitter has taken a stand here and I do think they should apply their policies evenly. Will they effectively apply this to everything or even have the capacity built out now to do so? I doubt it. They are a business who needs user engagement to drive profit from ads. If they constrain their most clicked tweets it could lower their revenue even if initially those tweets get attention for being removed.