Herein lies the rub. It’s extremely rare that a politician is tweeting about something so black-and-white that the tweet can be provably true or false. There is almost always a gray area between fact and opinion. In fact, people tend to vote for politicians because they agree with their interpretation and prioritization of a set of facts. So I think you’ll find the vast majority of political tweets reside in this gray area, because otherwise they wouldn’t be political in the first place.
And if “provably false” is the standard for editorializing, then Twitter picked a terrible example to set as the precedent. The tweet they “fact checked” was Trump making a prediction about the future. Namely, he was suggesting that mail-in ballots could lead to increased voter fraud. Not only is this an opinion, but it’s a projection about something that has not happened yet. By definition, it cannot be provably false.