I just got a 16GB M1 MBP, and I can see the HDR option in Youtube even on the inbuilt display. Didn't know that the inbuilt display supported HDR too. I thought HDR was going to be reserved for the future mini-LED displays.
This is done by physically increasing the backlight brightness and dimming the LCD around the video content to keep UI brightness levels consistent. The UI bit depth actually decreases once EDR kicks in.