Was anyone who played this back in the day on an N64 able to wall-jump on top of the castle?

No. (You?)

One of the things that was so magical about this game is that you never knew what the real limits were. Some things were just childhood fantasies, like getting beyond the waterfall, but other things like this actually were possible in retrospect.

Once... On an N64 attached to a 32" CRT in the late 90's an it was magical!

However was never able to jump over the flag pole in Super Mario on NES

This reminds me of pannenkoek2012's work on Super Mario 64: SM64 - Watch for Rolling Rocks 0.5x A Presses.

In particular, it makes me think about his "parallel universes" and QPUs.

Definitely. Bismuth's video is somewhat better at explaining the concept but it seems cruel not to mention Pannenkoek since his iconic video is what popularized the whole concept.

There's a clip from his video in the post :)

No that's a different video.

This post is about SM64 "parallel universes"!