That is not the smoking gun you think it is.
Again... SteamOS is just an immutable version of Arch Linux. That's what they are talking about in the article when they talk about turning off "read-only" mode. Being immutable makes it less likely to break/more stable, but doesn't "fine-tune" it for gaming.
Saying it's "fine-tuned" for gaming takes away from what is actually doing the heavy lifting for gaming on linux, which is Proton. One could argue Proton is "fine-tuned" WINE, but SteamOS is not "fine-tuned" for gaming.
What you're saying is false though.
You can read about what kernel SteamOS is using. The kernel itself is only like ~150MB, there isn't really a need to slim it down. I think, if anything, Valve contributes to the mainline kernel for linux.
Proton is the "fine-tuning", the OS is just Arch Linux with a paint job.