I agree geometry rendering is a fine choice for 4K in this case, since
Resogun is almost nothing
but geometry. That doesn't explain the issue, though. Geometry rendering makes the edges exactly as sharp as regular rendering, and edges are what you're checking for AA once it's scaled back down.
To be clear regarding what I'm talking about, here's a comparison. This isn't an identical shot on standard and Pro 1080p, but it's pretty similar. (I've blown up the shots 4x to make detail clear.) On the left, the standard version has decent AA (albeit occasionally scrappy, possible due to the fineness of some of the geometry). But on the right, Pro's supposedly-downscaled AA looks more like solid blocks of color along the edges.The blending effect is actually less smooth than on the lower machine.
The only thing I can think is that maybe VG Tech (who I got these images from) have flip-flopped their labels. If actually the left is Pro and right is standard, that looks much more like what I'd expect. Thing is, the claimed 1080p Pro shot is in an imgur album alongside definite Pro shots at 2160p, so such a mixup doesn't seem very likely. But stranger things have happened.