So it's either:
Ubisoft didn't want to put in the effort and made a lame excuse to have both versions at 900p.
or
MS didn't want more PS4 copies to be sold due to being the better version, and thus "talked" Ubisoft into forcing parity in hopes to have a more equal split in sales for one of the biggest selling games of the year.
I mean, they're bundling CoD, so they're good there. The next biggest franchise was AC, so they made a deal there, got that under wraps. It must be like a checklist of things to do.
It's a shame, because some of the recent videos (the ones showing their procedural engine tools) really had me excited for the game. But screw supporting these kinds of shenanigans.