A lot of newer MacBooks display HDR via macOS's EDR. This doesn't suddenly increase the bit depth of the display, but does allow the video to increase its peak

A lot of newer MacBooks display HDR via macOS's EDR. This doesn't suddenly increase the bit depth of the display, but does allow the video to increase its peak brightness values beyond that which the user has set. 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.