Well.. If you take London's argument to it's natural conclusion (or maybe just restate it?), that maybe it wasn't Halo specifically that sold Xbox's, but Halo "branded" the Xbox in a certain manner that lead to its sales.
That is what I would agree with. Halo made the Xbox the console version of PC gaming. It demonstrated that the console was essentially the console platform for 'hardcore' NA gamers.
People love to talk about Goldeneye and what a great FPS it was and how the game itself was better (according to a lot of people) than Halo was.. but it was Halo that brought FPS excitement off the PC, into the living room, to the masses of the NA audience. And then MS and third parties kept 'feeding' the Xbox those types of titles.
The reason the Xbox outsold the GC isn't just because of Halo.. but Halo branded the console as appealing to a certain demographic.. just like MarioKart branded the GC as appealing to a certain demographic.
(And of course, the opposite is also true.. that branding of the Xbox hurt/crippled it in Japan, and that branding of the GC hurt/crippled it among a certain demographic of NA gamers.)