TheChosen linked to you mean.
I actually doubt the footage coming from a working copy of the game outside of the original company. I just highly doubt this game ever left, I think Blizzard would have ensured it was deleted, destroyed and never to be found. As far as I think the footage is a clever staged bit of filming to make it look like he loads the game but then just shows an in company video of what the game was like, something they perhaps showed Blizzard before it got the can. It could also just be a copy of a demo version of the game, I remember it being advertised in small books that came with Starcraft and Diablo (one or the other I can't really remember) so it is a high possibility that a demo exists, note the game footage is not overly long. I could be wrong, it really could be out there like the guy claims, but why he hasn't sold it on and trust me their would be a lot of money in it baffles me.