> operating system only provides screen mirroring or app window mirroring on an external monitor

That's not true. It's probably written that way, because this is still an experimental feature so it is indeed not "supported", but it does work, you just have to toggle a few settings inside developer options.

And in this desktop mode it could make use of my 2k desktop screen, though it is quite buggy (it is a pixel 8 device, for reference)