If he's worked on "both Guitar Heros and Rock Band", I would wager he's a contractor, or temp of some sort, probably more design-oriented than engineering, and probably doesn't have the engineering experience to make such unfounded claims.
I suppose that a game like RB or GH might very well have most of its bottleneck performance issues around fillrate, however -- there's hardly anything else going on, gamewise. Fill is probably the bane of their light and shadow volume existance, since their game doesn't really do much else, from a technological standpoint. Having no other potential performance bottlenecks, and if you're unwilling to rethink your pipeline, or use deferred rendering, etc. to render your scene more effectively, you're bound to think that the 360 is the bee's knees.
From his limited perspective, I'm sure he's correct, or at least is convinced that he is. Ah the joys of working on simplistic games.
