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.
It shouldn't be. For the same reasons you would shorten TCP/IP to TCP, you should shorten GNU/Linux to GNU. This avoids all of the confusion about what is "really Linux", because when people say that they actually mean GNU.
There are reasons to want to talk about Linux, but it probably shouldn't be much more common than NT in day to day vocabulary.