The demos of upcoming Xbox One games at E3 2013 were running on a Window 7 based PC powered by Nvidia GTX GPU.
Check this photo which was shot at E3 2013 from NeoGaf boards.
Look closely at the card in the desktop. The demo units for the Xbox aren't using dev kits that are at the same level as what the Xbox specs are going to be. They are using PC's that have an Nvidia GTX card inside. For anyone that doesn't know about graphic cards and the technology differences, an Nvidia GTX 700 card should be much more powerful than the GPU in what will be the Xbox One.
Now read the tweet of Julian Rignall. 
