As I understand it, the assertion is that the 1080p FPS is the same as 2k/4k FPS, assuming that you have an infinitely powerful GPU. So the 1080p FPS is your max potential FPS at any resolution with the CPU, and then you need to look at a GPU 2k/4k chart to see how much FPS it can achieve from that target. HWUnboxed also reasons that gamers are not blindly using ultra settings, so in real scenarios people are going to be lowering their settings to try to achieve a specific FPS target anyway. They also mention that lowering ingame settings doesn't usually affect the CPU FPS benchmark.
So in summary, the 1080p CPU benchmark is the ~highest possible target you can achieve, and then it's up to your GPU and ingame settings to decide how much of that target you can reach. It's a little more difficult to grasp and calculate mentally, but it prevents the 2k/4k benchmark data from showing what is effectively misleading "point in time" data that will not be useful if you have a different GPU or ingame settings. This is most clearly demonstrated by re-reviewing older CPUs in the future-proof section and showing that putting massive GPUs on old CPUs puts the FPS benchmarks of all resolutions to roughly the same value - i.e. the CPU doesn't truly have an effect w/r/t resolution, it's mainly just the GPU.
In my experience, there is nearly no difference between windows and linux when it comes to piracy. There are a few games that linux can't run (anticheat), but generally that shouldn't be an issue for games that you would typically pirate. Linux does have a standard learning curve though, and you'll need to get familiar with Lutris or some other Wine prefix manager to manage your games. If you're dedicated to moving to linux, game piracy should not be a deciding factor.