The video above has been making the rounds for a while now; it's a safe bet that most viewers have assumed it to be yet another made up mash-up and nothing more. I believe Dorkly was the first to make that joke.
But upon further investigation, turns out the footage is actually proof of concept of a very real game. Mari0 is not a rom hack or the like but an original title created in Löve2D. Upon completion, it'll include levels from SMB plus part 2/Love Levels, along with original levels that have "portaly puzzles" and tell an original story.
Furthermore, simultaneous multiplayer will be supported, though the most intriguing promise is how the "level editor will be embedded in the game so you can edit the level while you play". As for when it comes out, the creator has take a firm 'it's ready when it's ready' stance.