I see. As I said, it was a bit hard to make out in the video.

In fact, Windows should have had an easier time encoding

Granted, I don't know too much about AMD's video encoding solutions, but from a cursory glance on the internet, it seems like their H.264 solution is quite bad compared to H.265. Given that the game is GPU-bottlenecked and your CPU isn't stressed at all anyway, I'd recommend recording these tests using the CPU to eliminate more variables.

Beyond that, the methodology is not flawed, if you can even believe that.

Well, yeah. As much as I'd like to believe, these differences are way too big for me to do that, even with everything you've shown in the video. Occam's Razor would suggest that it's much more likely that the benchmark/setup is simply flawed in some way, rather than multiple teams of OS-, hardware-, and game developers not realizing a gigantic 25% performance improvement on the table that's somehow more or less "accidentally" fixed just by using Linux/Proton/DXVK.

Not saying you're wrong, but it'd need a good chunk more evidence for me to believe that.