"Steam Deck" is clearly not "Desktop Linux", just like Android isn't, or PlayStation isn't "Desktop FreeBSD". It's reasonable to make these distinctions. No one was talking about "operating system kernels".
[1] https://www.digitaltrends.com/computing/how-to-use-desktop-m...
It's not "vanilla", but so isn't Ubuntu, Linux Mint, Fedora Sericea, or whatever else. The underlying stack is the same, there's just customizations on top.