Dimming the entire display is not how HDR works especially on OLED phone displays.

If you are not outside in bright sunlight it’s almost certain that your screen is not at its highest brightness in the first place.

All HDR is doing is adding more range between the darkest element and the brightest element.

I.e., if you go to this webpage and set your screen brightness to maximum, the HDR effect visibly goes away because you’re already telling your screen to maximize brightness everywhere. Put the screen brightness at 50% and the HDR effect is visible: the bright parts are brighter.