Maybe because "being compatible with Linux" doesn't mean anything specific. There's no spec so nobody can follow it... which is what GP is saying I suppose: Define a bar so people can try to aim for it.
Wouldn't "Compatible with Kernel 5.11" be a pretty good/accurate bar? That is if they upstream their drivers or develop their closed sourced ones against certain kernel?
Except that when you get the device in your hands, the kernel version advertised is in the very best case one year old. So irrelevant.