Why does playing super mario bros on google make a run illegitimate? I think it should be legit if you don't have any other way to play the game.

Because it isn't an accurate emulator compared to the ones that are allowed in the game rules.

If you’re talking about one of those online emulators, it’s because the emulator is completely inaccurate, and the only good one I can find doesn’t let you disable l+r