There is no proof, hence the additional debate.
But the problem is the things we know:
* Ubisoft said it's CPU bound, spouting AI bullshit, but not mentioning GPU
* The Xbox One is considerably (40%~50%) weaker than the PS4
* They run on very similar architecture
* The only games that match parity have maxed out at common thresholds like 720/1080p and 30/60
* The game runs at 900p/30 on both devices
* 900p is NOT a threshold
* 1080p isn't a huge jump from 900p.
* Microsoft has been at the forefront of AC:Unity marketing
Combining all of that, we know that a device that is vastly more capable than the other should be easily able to make the jump from 900p to 1080p. No problem. Why pick such a weird resolution for the PS4 when it obviously is powerful than the XB1?
It's pretty easy to figure out the issue.