They're just mistaken, RTLinux is hard realtime. It works by running a tiny real time OS on the bare metal, and then running a Linux kernel as a (lightly) paravirtualized lowest priority task. You write your hard realtime code to run on the RTOS, and then your soft/non realtime code runs on Linux.

In this model, your compositor, input pipeline and (at least the UI latency sensitive part of) your applications would have to be ported to the RTOS, which makes this pretty infeasible. But it works really well if you have a some hard-realtime control loop talking to a bunch of non-realtime network IO or UI code. Would be a fun way to build a wifi controlled quadcopter.

These days you probably want to use Xenomai, or possibly RTAI.