It's to show that a person with some spare time and a little above average IQ can figure out something beyond people of massive authority, on the very subject of their authority, even if that subjects is as scary as QM

That's one theory. Another is that he read Deutsch, and found him persuasive, and read .Jaynes and found him persuasive, and noticed a nice consilience between MWIs determinism, and Jaynes's probability-is-in-the-mind theory,