Sure, it's common knowledge that games are usually run on dev kits, however the card in question is not a close approximation of the Xbox one. The performance gap between the GTX 680-780 and the AMD 7770 is massive, that is the problem people have here. The issue isn't that they were using PC's to show off games, it's that they are using a graphics card with literally twice the performance potential, and no DX11 api/Windows isn't going to crumble the performance of a 780 down to the point it performs like a 7770.
If they were using PC equivalent parts nobody would have a problem but this is like showing up with a Ferrari Enzo, when everyone was expecting you in a Nissan 370z. Nevermind the fact they weren't even using AMD hardware
As pointed out by an actual developer -
And the guys over at Beyond3d are saying the same.