Not exactly because it is still being scaled by your brightness setting. As in, start playing an HDR video and then mess with the brightness slider. You will still see the HDR content getting dimmer/brighter.

It's easier to think about in Apple's EDR terms. 0.0-1.0 is the SDR range, and the brightness slider is changing what the nit value is of "1.0" - is it 100 nits? 300 nits? 50 nits? etc... HDR content (in theory) still has that same 0.0-1.0 portion of the range, and it's still being scaled. However it can exceed that 1.0. It's still being scaled, it's still "respecting" that slider. Just the slider wasn't a brightness limit as you're wanting it to be, but a 1.0 alignment point.

The problem comes when HDR content is disrespectful to that. When it just absolutely slams the brightness, pushing all of its content way past that 1.0 value. This is bad content, and unfortunately it's incredibly common in HDR media due in part to the fact that the original HDR specs are very incomplete and in part because it's a new loudness war.

You have an "old" style handling of HDR on Android. Newer/better devices don't do that (specifically those that support https://source.android.com/docs/core/display/mixed-sdr-hdr )

Similarly MacOS/iOS doesn't do that.

> Not just brighter, but also darker, so it can preserve detail in dark areas rather than crushing them.

It does not get darker, and while PQ allocates more bits to the dark region HLG does not. And, more importantly, neither does the actual display panel which are still typically gamma 2.2-2.4 regardless. So PQ's extra precision in the dark areas is ~never utilized other than as tonemapping input, but the resulting output does not have any increased precision in the darks over SDR.

In fact it actually has less precision in the dark areas as the increased display luminance range means the panels native bit depth need to cover more range.