Depending on how this thing is implemented, the claim in the title might or might not be true. For all we know, CarPlay might just be an extension to the existing car entertainment systems, using something like VNC (or hopefully something more optimized for the use-case) to show the iOS screen on the existing infrastructure.

In that case, the car is running QNX because it has always been running QNX and because the car must be useable even if the user decides to switch to a different platform or loses their device.

In that scenario, saying CarPlay is running QNX is similar to saying your Thunderbolt display is running OSX when it's connected to your Mac running OSX, or using an even closer analogy, similar to saying that your OS X machine is running linux because you're using SSH connected to a linux box (or any other kind of remote desktop)

Personally, I would guess this is the kind of integration at work here. This is not about actually delegating the entertainment system functionality to iOS. This is just about sending touch events and maybe telemetry data to iOS while displaying the video feed produced by iOS.