The silliest part of that conspiracy theory is that, in theory, Ubi stands to make more money off each PC copy sold than console copies sold, because they don't have to fork over the licensing fee. I'm not saying that conspiracy theory is definitely wrong, though.
My own theory is less conspiracy and more incompetence/rushing out the door. I'm guessing what happened is that when they first released the video at E3 Ubi really wanted to make the game look like that, but the hardware wasn't completely finalized for the PS4 and XBone at that time. Then when they got to work with the finalized hardware, they realized that the miraculous new systems from Sony and Microsoft couldn't handle all the extra shiny, so they swept some of the extra shiny under the carpet and worked on a gimped version as a base across PS4, XBone, and PC, plus working off a lesser base likely made it easier to do the PS3 and 360 versions, though those still turned out like garbage, from what I read. Then they spent most of their time up to release trying their best to get stable framerates on the PS4 and XBone, and trying to fix the dog**** framerates on the PS3 and 360, and either ran out of time to reimplement the stuff they swept under the carpet for PC (which would also explain why the PC version is so poorly optimized, they ran out of time to do that too), or they just flat out forgot about it.