Sure, but the video decode IP is a different piece of hardware from the video scaler/compositor. In a typical SoC, you have the GPU and the video decode engines as separate blocks that feed into the video scaler/compositor which then blends, rotates, scales, .. etc the inputs and feeds the final signal to LVDS, HDMI or whatever.

Now does it make sense to have your video decoder support 10 bit when your compositor can't handle that format? I guess you could argue that 10 bit source material will still improve the quality even if you downsample it to 8 bit for display, and I think for example DVB-T2 has standardized on 10 bit so if you can't decode that, that's a large chunk of market gone.

Ultimately, from the little we know, the compositor hardware is the same as the old Rpi, and that couldn't do 4k60 and it can't do 10 bit depths. The Raspbian image currently doesn't use the Linux kernel implementation for driving the compositor (it uses the fkms or "firmware kernel mode setting") so there might well have been hardware tweaks.

(Ok, instead of making this comment thread any longer, I simply asked and the compositor hardware in the RPi indeed has HDR support:

https://github.com/raspberrypi/linux/issues/3020#issuecommen...

Tweaked from the old HVS in the old RPis.)

Yeah, that makes sense - you wouldn't want to gimp the system by only doing half a job :)