To be fair to the 'CPU bottleneck' crowd, doesn't resolution also affect frame rate? In that, if the CPU is causing the framerate to be locked up to a certain point, wouldn't increasing the resolution subsequently lower the framerate? Not saying I agree with the Ubisoft reasoning, but stating that the CPU bottleneck refers exclusively to framerate fails to include all of the factors.
If a game is CPU bound due to the AI and NPC number requirement (as it is the case with this game apparently) increasing the resolution will not cause the CPU to be overloaded with these specified task. If it was to process animation, pathfinding and information for 4000 NPC and the AI for them then that would not change due to an increase in resolution.