Don't forget that one of the design goals (when John Carmack built Tech 5) was to be able to alter and/or design the game across all platforms at once. So while you're right the different platforms have different code the software Carmack created translates across the three. So it is possible that this demo could be running from any of the three platforms. Though I tend to believe it's on a PC just using a controler.
edit: Oh, add me to the list of people that had written this one off but after watching this video has had a change of heart.