Confirmed by Jonathan Blow and Sucker Punch, demos were running on PS4 devkits. Most importantly, they were running PS4 GameOS, not Windows.

Blow: We worked very hard to get our game running on the actual PS4 hardware and operating system in time for the show. As did many other devs.

Sucker Punch’s Jason Connell added:

Yup, we were definitely on a dev kit. [For inFamous: Second Son]
For the MS apologists, if that was an XBone devkit, why was it running Windows 7? No matter how much spin is given, it still looks just as bad.