The internet is those thousand monkeys all on typewriters, just scaled up several magnitudes so enough shit gets said that something eventually ends up being a prediction of something else. Topical matters like a controversial presidential candidate in an election year are bound to produce a shit ton of these kind of "prophecies" that too many will jump on as some kind of revelation despite it being a drop in an ocean of bullshit.

Yeah, but the scam is usually to be pretty vague. 'There will be a failed assassination on Trump, he will be hit in the ear by a bullet, he will fall down' is a pretty risky bet, if you're just bullshitting.

Putting aside the options where he's a true prophet OR he's a true believer but he's wrong, scamming something like this is not as risky as it seems, because if it doesn't happen, who would have heard of him? Who would go back and say "Look at this guy, he was bullshitting!" And, even if someone did, who would care? The guy would say "Yeah, I had a dream, I thought it was prophetic, turns out it wasn't, oh well, no one bats a thousand." Then he'd point at some of his other predictions that look good and emphasize those. The other big thing is that in this case it turns out he was really on the money, but that doesn't mean he had to be for it to work. With any assassination attempt that got anywhere near, he could say "look, he nearly got killed just like in my dream," and of course "someone is going to try to kill Trump and make a decent attempt of it" is a much safer bet.