I'm an engine programmer who has been lead on 2 published titles (PC not console).
I'm pretty sure they thought of that too and already did it. One has to write multi-threaded code for these consoles since they are multi-cored and otherwise most of the resources are wasted. Multi-threaded code is really easy to arbitrarily set tick frequencies and lock contention on the rendering thread is actually lower when you set the tick frequency of things like physics and AI to a lower frequency to your FPS, especially if your FPS is not an even multiple of this frequency. We run Havok at 50hz and render at 60fps, it sounds counter-intuitive, but it looks and feels great.
The things is though, this game is obviously either GPU limited or close to becoming GPU limited. The key here is not the 30fps, which without looking at profiling results, could be equally easily explained by CPU limiting or GPU limiting, but the resolution of 900p, which the CPU should have absolutely nothing to do with. So you cannot confidently say that it is GPU limited now, but it certainly would be at 1080p, otherwise they would have just upped the res without it slowing the framerate.
The issue here is Vincent Pontbriand is probably not a technical guy. Roles between companies vary and it's hard to know who if anyone reports to a "Senior Producer". If the engine programmers reported to him, it would be possible that he was lied to, since explaining exactly why framerates are the way things are is often tiresome and complex, since bottlenecks can be in many different places in the GPU pipeline (geometry, shader, input, texture, ROP, framebuffer) and the position of the bottleneck may shift while rendering a single frame, the bottleneck can also shift between the CPU and the GPU during a singe frame if the instruction buffer fills up. As it is, they probably don't report to him, so I would say that he probably just doesn't know the whole picture.