From the article:

All of the current titles within the Steam client are using GLX.
This isn't strictly true. Most of the current titles are using SDL, which uses GLX behind the scenes. One could swap out the game's included SDL library with a build that targets Wayland or Mir, and the game would run without an X server and not know or care that it's not using X11. This can happen without updating the game binaries directly.

For games that don't ship their own copy of SDL (and thus use the one included in the Valve Runtime by default), you don't even have to touch the game's depot at all. Valve just needs to update their Runtime package and it's good to go.

SDL is smart enough to try to use Wayland or Mir first, and if they aren't available, use X11, so one copy of SDL will work on whatever the current system has.

This is an oversimplication; there are probably games that use SDL but will cheat and try to talk to an X server directly for various reasons--but this is rare--and there's at least one big game on Steam that's talking to GLX directly without using SDL. But by and large, switching the Linux Steam library from X11 to Wayland and/or Mir is not only doable, it's probably relatively easy.