Notched MBPs can't do HD through resolution changing. (I would love to be proven wrong on this. See my other comment for hoops I jump through to get around this.)
I can set my display to 1728x1080. That's HD.
I can also output my Mac to an HD TV, Projector, etc. And I have the notch on my MacBook Pro.
Do you have some other definition of HD that I'm not understanding?