This is totally false. I did it, legitimately. I went an beat the entire story mode, on Hard (Not Super Hard), with every single character, one week during the summer last year. I also did the "Win 1000 coins in Game Guy's Room" thing that sometimes accompanies the claim, and my mountain is still not gold. Very sad, to be true. But, it was the journey that was fun, and the reward doesn't mean too much to me.
I can post proof pictures if someone requires it, but I'm here to tell you, definitively, that the Gold Mt. Mariomore DOESN'T exist.
Edit: S ranks on every board, as well. It was quite the journey, let me tell you. A lot of resetting and manipulation was required, but even with that, it was quite tough.