Many computer users run a modified version of the GNU system every day, without realizing it. Through a peculiar turn of events, the version of GNU which is widely used today is often called Linux, and many of its users are not aware that it is basically the GNU system, developed by the GNU Project.
There really is a Linux, and these people are using it, but it is just a part of the system they use. Linux is the kernel: the program in the system that allocates the machine's resources to the other programs that you run. The kernel is an essential part of an operating system, but useless by itself; it can only function in the context of a complete operating system. Linux is normally used in combination with the GNU operating system: the whole system is basically GNU with Linux added, or GNU/Linux. All the so-called Linux distributions are really distributions of GNU/Linux!
Yes, there technically is a Linux kernel, but if it's "just Linux" then macOS is "just FreeBSD", because grep -V tells you so, because it has dtrace, because you run (ran?) Docker with effectively FreeBSD's bhyve, etc.
If you wanna spin it even further neither are Safari and Chrome or any other Webkit browsers just Konqueror because they took the layout engine code from KDE (KHTML).
And you can totally install Debian and even OpenBSD, etc. on a Steam Deck and at least the advertisement seems to indicate it won't be all that different for the VR headset.