I think the place where this falls apart is that he's talking about future events. It's a prediction about the future. Unless you believe he's insinuating supernatural abilities to tell the future exactly, he's just expressing his personal level of confidence about his prediction... which is an opinion.

The real problem here is calling what Twitter did a fact check. Because what Twitter did was just promote opposing opinions/predictions about the same future event. Neither side can make statements of fact about the future, they can only state their opinion and express their level of confidence in that opinion.

People are held accountable for comments on future events all the time:

- medical claims - fiduciary - threats

If a doctor tells a patient that there is no way (ZERO!) that taking “the hydroxy” will be anything less than substantially successful - do we let it slide because there is a chance it might be true?

Of course you should hold him accountable. Vote against him. What does that have to do with Twitter "fact checking" something that isn't even a statement of fact? Do you think they'll start fact checking things like statements of fact about the gender pay gap, perhaps the most debunked concept in all of economics?

If you're going to autistically parse the President's tweet, then I will gladly point you towards the word "substantial" in the President's tweet. How do you intend to gauge the President's internal thoughts regarding what he believes would be a "substantial" amount of fraud? He might feel a single bad vote is substantial.