I don't know the specifics of why Nvidia does not work with both, but I would avoid post-processing because the code I see that implements that in the engine looks like it just an inferior way to acheive anti-aliasing as what it does is to take a copy of the current frame and a copy of the previous frame and blend the two together. So what you get is a slightly smoothed edges, but also slightly blurry image. I think the effect was put into the game back then because not all GPUs supported AA back when the game was first developed. The original also did not even support AA and I was the one that added that as an option; I just left PP as an option just because I wanted the 32 bit to still support very old machines as far back as WinXP.
My suggestion is to ignore post-processing and just use AA if you want smoothed edges.