SteamOS is actually Archlinux which is a mere Linux. Archlinux can be considered as clean sheet Linux, they avoid patching and modifications if possible. So probably even the FSF people would accept it as GNU/Linux?
Even more. The SteamDeck is using a X86_64 CPU and RDNA2 GPU. No hidden firmware locks. So it is an IBM-PC. It counts therefore 100% as Linux. I would even say 120% because it comes pre-installed.