Custom kernels aren't commonly used, it's a niche within a niche. They literally don't need to "tackle" anything here, they can just support the Steam Deck and call it a day.

The biggest legitimate concern here is cheating, since cheaters could conceivably customize their kernel to bypass whatever EAC does. That's a theoretical attack only, and there are plenty more theoretical attacks that aren't unique to Linux.

It's not an issue of compatibility, but Tim thinking he owns your computer. I don't want any software that needs to control my computer; if it can't run in a sandbox, I don't want it. I understand him worrying about a base level of compatibility, but I don't have any sympathy for the "but Linux is so diverse" argument, officially support one and the rest will figure it out.