The plot leaked in full about three months before the film released. And saying that Palpatine will die doesn’t exactly make you Nostradamus.
I remember he mentioned a source in the video, I don’t remember if it was a full script leak. But those aren’t always reliable either, wasn’t one of the prequels leaked too? But that was just an example, the how he died (blowing himself up) because i remember everyone saying that sounded dumb. Did the leak have the Falcon shot down?