Since they do not alter the content of the tweet itself in any way, and since "fact check" link itself does not express an opinion (eg: it just says "get the facts" as opposed to "potentially misleading") then it seems extremely hard to arge that it's editorialising of the tweet. If the fact check page contains very opinionated content written by Twitter employees then it might bring it closer into consideration, but if the fact check page factually summarises and cites external sources again it is hard to argue.