I think there might have been an LW comment somewhere that put me on that track or maybe even outright suggested it

I certainly made a remark on LW, very early in HPMoR, along the following lines: If magic, or anything else that seems to operate fundamentally at the level of human-like concepts, turns out to be real, then we should see that as substantial evidence for some kind of simulation/creation hypothesis. So if you find yourself in the role of Harry Potter, you should expect that you're in a simulation, or in a universe created by gods, or in someone's dream ... or the subject of a book :-).

I don't think you made any comment on that, so I've no idea whether you read it. I expect other people made similar points.