I think it's legit (and awesome). I've seen/heard manualists who were that good in the past, and really, making those sounds with a human mouth would probably be even harder than making them with a pair of hands. Especially the final squeak!
On a related note, however, that Blindfolded Pianist is a pretty cool guy. I emailed him a couple years ago (I guess around the time the first video surfaced), and he sent me a bunch of .pdf's that he made himself of Mario sheet music. It's all great, and I revel in my newfound ability to rip out the Mario theme on the piano whenever I please.