So in court there's the concept of hearsay, which generally makes certain statements inadmissble as evidence. The classic example is me testifying "Alice said Bob told her he did it".

One of the exceptions to hearsay is called the admission against interest. That means that if you say something that hurts your case or hurts you in some other way (eg implicating you in a crime) then you will generally be allowed to testify to that.

So this isn't a court of law obviously but I still find this analogy useful. Yes, Trump says some crazy stuff and even openly lies. All of that's true. But that doesn't mean you should ignore everything he says. What he says can be corroborated (or contradicted) but it also carries weight if it's an admission against interest.

In this case, Trump claiming to have armed "protesters" is absolutely an admission against interest. It undercuts American propaganda that the Iranian regime brutally crushed an organic protest by ordinary citizens. As such, at least for me, the statement carries more weight. You can still look at the statement and see if other evidence contradicts or suports it of course.