This is true, but you still have to position the window on the screen, you just end up doing it manually by forcing exact pixel coordinates. IME it's annoying enough then I'm surprised nobody's made... "fullscreenwm", a <1KB executable or perhaps even shell script that fullscreens the only window on the screen and then blocks forever. Or perhaps it exists or is such a trivial application of some window manipulation command line utility that it's unnecessary and I just haven't heard of it.

You might like nullWM.

https://github.com/DebianJoe/nullWM

   xdotool getactivewindow windowmove 0 0 windowsize 100% 100%

should work. At least for the fullscreens the only window part.

For Wayland the cage compositor does the same thing.