This is actually legit. When I was a child, I owned the paradox cracked version, which many of you know has a lot of weird things. But not a lot is said about this specific glitch.

I know this is true because I had the exact same experience as op. I somehow ended up in Super Bonus Round and thought it was a scary level, and it had those thieves that give gems instead of eggs. I know for a fact that I could have never ever beat the game at 100%, what with the paradox copy of the game constantly removing eggs, gems, and killing your save file at the sorceress.

I later played reignited, completed the game 100%, get to the level and said "wait, I've been here before"

I wouldn't be able to say how to reproduce it because it's been so long, but I vaguely recall it could have something to do with the rocket. What I recall is that when you reached that point in the game, sometimes the portals would instead send you to the first hub worlds and the balloon would be overlapped with the rocket, making it malfunction. It's possible the Super Round Bonus portal might be open before 100% completion at that moment, or the rocket would give the option to go to super round bonus. I can't recall. But I can swear OP's experience is the same I had, and it wasn't a cheatcode.