I first heard about this story about 24 hours ago, so X-S was a little late getting it, but that's not the point. The point is, IGN did post it and the fact remains E3 is an industry event and thus the chances of this being true make it more likely than anything, really.
Also, there's been no mass amounts of news bits coming out PRO-360 ANTI-PS3 from "annonymous sources" at MS -- that's all your imagination. The only other news item from an annonymous source was these pricing rumors of the new 360 peripherals.
And, btw, I saw this time and time again ON G4 -- They showcased it on their Live G4 coverage each of the 3 days they had their 3-hour Live from the showfloor coverage and said MANY times that it's actual in-game footage.
The last comment I heard a G4 host mention about it was last night when fans rated it "HOT" as opposed to "SCORCHING" and/or the highest -- "VOLCANIC" ---- And I'm 99% sure that they said fans probably didn't rate it too high because it was only of one level, the collisium (many ways to spell that), and a lot of hack and slash.
The last thing I'll say is if it really is in development for the 360, too, which many believe and it has been rumored way before E3 anyway, it only makes sense it could have been running on a 360. Maybe Ubisoft didn't have the PS3 version ready for E3 so they used the 360 version of the game to show off, but wanted to hide that fact -- That is VERY feasible.