But the memory contents aren't actually being modified in an unintended way. They're only being used in an unintended way. The random number generator is supposed to tick this way, and the sprite data is supposed to work this way. The key exploit is in getting the game to run a subroutine at the wrong time, so that it uses the valid memory contents as code and skips to the end.

There are other mario games where you can go out of bounds into memory and start flipping bits. That is memory corruption. Here I don't think the term applies.