If you subscribe to the vastly-oversimplified concept of a multiverse, then you must believe that, given an infinite set of potential universes, all possible things can and must occur in at least one world parallel to our own. Which means that somewhere, on some alternate version of Earth, Super Mario Bros. stars a textured-yet-pixelated biker named Manley who is trying to track down his kidnapped motorcycle. Kidnapped, that is, by aliens.
To the game's credit, it's completely up front about what it is: "This retro platform game pays tribute to, and parodies, classics such as Super Mario Bros., CastleVania and Mega Man, in style!" It doesn't try to hide behind feigned originality.