I think the prior posters are referring to the fact that CarPlay supports other map apps, which is definitely true, but may be missing your question as to whether other map apps show up "in your car's HUD." On my BMW, I can use any CarPlay-capable mapping system (Google Maps and Waze are the others I've tried) in the CarPlay window. However, only Apple Maps also provides information to the car's HUD; for example, displaying turn-by-turn directions on the main cockpit display behind the steering wheel and on the heads-up display (HUD) on my windshield.
I don't know whether that's due to an Apple restriction or not, but I think this may be what OP was asking about.