kdm does not work without a locally installed X server, so it should RDEPEND on this.
It depends on whatever in needed to compile...
Besides, kdm (and entire kde) works flawlessly with several X-terminals for me, without any X server installed. Not every setup is a primitive as Joe User's machine.
The purpose of RDEPEND is to pull in *run-time* dependencies, of which one of kdm's is a local X server. kdm will error starting if X is not installed.
You can install kdm and gdm on a machine that's headless and remote into it for logins. This would force your headless machine to contain a local X server, which would be improper.
^^