Yes, it is. Mankind's amassed knowledge of physics, chemistry, and astronomy has lots of evidence to demonstrate why that would not happen, and when it probably will. That can be compared to the evidence you have to show for your claim, which is nothing.

But that's entirely his point. Facts can be backed up by evidence. As you rightly pointed out, he can provide no evidence as proof of his wild prediction. We can make predictions based on scientific facts, but a prediction itself is not a fact.

Some predictions are known with enough certainty that they would be very accurately called facts. Example:

Fact: The sun will rise at 5:50am in San Francisco tomorrow.

False: The sun will rise at 10:00am in San Francisco tomorrow.

It certainly seems that mail-in voting is more easily corrupted than going in person and showing ID and voting in secret.

There are tradeoffs with accessibility, of course, but it seems depressingly easy to sell a mail-in ballot to someone else or to be verifiably coerced by a boss or family member into voting a certain way.

All-in-all Trump tweets worse, dangerous lies on a daily basis so it's bizarre they're pushing back now on this.

If we're just thinking of ways voting can be corrupted, in-person voting can be coerced too. Boss forces you to take a photo of your ballot as you vote in the booth, etc.

In reality, several states have had mail voting for years- Oregon for decades- without coercion being a problem. Millions of votes cast without the problem you suggest. It's not a problem, despite using our imaginations to come up with ways it can be.

> Boss forces you to take a photo of your ballot as you vote in the booth, etc.

You can take a picture of the ballot with your boss's choice but then spoil it and get another one.

> several states have had mail voting for years without coercion being a problem

How do you know this? How could anyone know if a wife voted the same way as her husband because she wanted to or because he just filled out both mail-in ballots himself?

There are differences in scale too; It's hard to steal a ballot, for example, when only 1% of the population is voting by mail. It's easy when 60% of mailboxes have a ballot in them on the same day.

Finally even if it hasn't been a problem so far, it is not a "fact" that it cannot and will not be a problem in the future.

> Finally even if it hasn't been a problem so far, it is not a "fact" that it cannot and will not be a problem in the future.

Twitter said no such thing, however. Twitter's annotation simply noted that Trump's claim was "unsubstantiated". They did not make an assertion of the inverse of Trump's claim.

> How could anyone know if a wife voted the same way as her husband because she wanted to or because he just filled out both mail-in ballots himself?

This could be easily said about in-person ballots too. 8 states require photo ID, and many people will be covering their faces when they vote in-person anyway.

This is the point that most people seem to be missing and this is where Jack Dorsey seemingly made a huge mistake. I don't know if he did it on purpose to force his company to do something he knows should be done, but he doesn't have the will to see it through, or if it was just a really bad decision.