But, everything he has the characters do is something he knows, and it must be that way.

No, no... that's true in a limited way, I mean the characters don't know about any real-world experiments that he doesn't, for example.

However, they can have novel insights. This is a good reason to write fiction, in fact. I write dialogues between characters in comics that I read when I'm stuck on math or programming problems. When I do this, it's still me that's thinking of it, of course. But it comes to me in a character's voice.

So that might happen with HPMOR, too. There might be things in HPMOR that Eliezer only figured out when he heard, internally, one of his characters say them.