[1] Case in point: glibc's compatibility guarantees are weaker than what you get on Windows. (For instance, your system's glibc cannot be older than what a game is built against, which may present problems for devs using Fedora/Arch and players on Debian/LTS Ubuntu, something I've experienced first-hand for my apps.) The X11 to Wayland migration is also still underway. (Though things are getting better, the attitudes of some Wayland maintainers are a bit concerning: "I don't [care] what you think is normal behavior for games. You get certain guarantees with wayland. Deal with it. If clients decide to do exactly what they do on windows or X11 they won't work correctly." [3] I'm not sure game developers would enjoy such reception.)
[2] https://store.steampowered.com/hwsurvey
[3] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18...
Porting games from Android/NDK into GNU/Linux is relatively a child's play.
Playstation OS is also POSIX friendly.
Finally every serious middleware engine supports GNU/Linux.
Still the amount of studios that care about GNU/Linux is almost zero.
With Valve, there are no reasons to bother at all as a studio, target Windows/DirectX, let Valve do the work, collect the money with zero additional effort.
Now with Windows based handhelds, Valve will learn what happened to netbooks.
What do I need to make a video game? Input, sound, graphics. What use is a shell, a userland, or even a C library (if I'm not writing in C) for making a video game?
Android is _not_ GNU/Linux. It doesn't need to provide a shell or userland as part of the platform, and its C library is bionic not glibc. It also provides a lot of things GNU doesn't*, like input, sound, and graphics. It's also not designed to run on a desktop, and the differences between mobile and desktop are non-trivial and slowly growing.
Windows and macOS are full graphical operating systems. Linux is just a kernel. GNU/Linux is just a command-line operating system. Great for servers, terrible for video games.
Moreover, commercial video games are distributed in binary format, not source. Even if input, sound, and graphics were solved on Linux in a consistent way, the developers** can't help but break ABI compatibility every couple of years. Many apps from Windows 3.1 days can still run on Windows 11. What binary from Linux's early days can still run today, never mind with graphics or sound?
* = You could include GLib/GTK+/GNOME but then you're targeting a specific desktop environment and not simply "Linux" or even "GNU/Linux" anymore
** = Except for the kernel developers, upon whom Linus forces "never break userspace" as a hard rule
I believe pjmlp's point (although it requires a fair bit of reading between the lines) is that Windows already has fantastic backwards compatibility (as you elaborated on), and Valve's work has created a situation such that all developers need to do is target and build for only Windows, release Windows-only binaries; then, Valve/WINE will do the hard work in ensuring they run seamlessly on Linux. This means developers don't need to care about building natively for Linux (à la Factorio and a tiny handful of other games). In other words as another commenter said, the real stable ABI on Linux is Win32 + WINE.
Furthermore, Valve's work also negates the work of open-source engine and game developers who have ported their engines and games to native Linux. This is because developing for Windows is a known quantity, and there is an overwhelming volume of resources, effort, and experience in writing games for Windows.
pjmlp concludes with 'Now with Windows based handhelds, Valve will learn what happened to netbooks', which I gather to mean that the Steam Deck will lose popularity to Asus and MSI's (and soon, other manufacturers too) handheld systems, since running most games directly on Windows is still easier than the occasional faff that someone has to do when running games on WINE/Proton.