Comparing DC to GC is little different to comparing it to any other of that generation, maybe GC didn't sell the best but it stuck around the whole gen unlike the Dreamcast that was abandoned before GC even came along so their underdog status is nowhere near each other.
GC sold considerably closer to the Xbox worldwide, on par really, than the DC which shifted less than half the units. How's this comparable underdog threshold somehow enough to fit the GC but not the Xbox over just a few million units more?
Just because Xbox was technically second to the PS2? It left everything so far in the dust it couldn't see them any more so in that the three are all underdogs and much closer to each other than any of them is to the PS2.
So, this makes no sense as anything more than revisionist history trying to make the Xbox appear on par with the PS2 or something else equally weird, I have no idea what even.