Literally the first sentence in its Readme says it is.
The README is one thing, in actuality its another. I’ve personally tested it with Arch Linux KDE on both my Surface Pro 7 and a desktop PC with an Acer touch-capable monitor:
There is no Input Devices -> Virtual Keyboards section in KDE System Settings when running in X11, so you can’t even enable Maliit for your X11 session.
There’s a lot of situations in X11 where tapping on a text box won’t trigger onscreen keyboards and since there’s no way to manually trigger Maliit to appear like you can with the Steam keyboard with STEAM + X, it is kinda broken.
By all means, feel free to tinker. You probably need to disable the read-only filesystem on SteamOS in order to properly install the Maliit package. All I am saying is don’t expect an experience that’s usable in SteamOS’s X11 desktop session.