The ordinary brightness slider changes the backlight brightness no matter if HDR is enabled or not. However when HDR is enabled (at least on my monitor) the brightness of the backlight only limits the maximum brightness, and (aside from backlight bleed) doesn’t affect the general screen brightness unless it’s below the SDR brightness.
It would be nice if the SDR brightness and SDR color intensity sliders were available in that menu, or at least if the brightness slider controlled SDR brightness instead when HDR is enabled.
Less importantly, the backlight brightness could also be adjusted automatically depending on the actual brightness of the brightest thing on screen to save power, but I’d think that’d be harder to implement and calibrate