Who gets to decide what's disinformation?

The leader of course.

And we know she is good because all the news media say she is good.

And we know the news media is good because she tells us it is good.

Welcome to Hotel California, you can check out at any time, but you can never leave.

We don't need cosmic deity levels of certainty. Most first-world judicial systems are predicated on the standard of evidence being "reasonable". As in "reasonable cause to believe" or "beyond a reasonable doubt". We have functioned pretty well with based on "reasonable" and we can apply that same standard here. It is fair to label a statement as at least misinformation if it seems to be untrue beyond a reasonable doubt. Disinformation requires intent which is much harder to determine but not really relevant.

But Brutus says he was ambitious;

And Brutus is an honourable man.