That can't be right. I'm reading this comment on an HDR monitor over DP right now.

Don't all USB-C video outputs use DP alt mode too, with an HDMI adapter at the end? And they can do HDR.

The cable length limitations are also a pain in the ass for not-uncommon A/V system configurations. 6' recommended max, and the best you might get working stably if the device and cable gods smile on you is 15'. 6' is the lower edge of acceptable for just about any A/V system setup (in practice it means your devices need to be within about a meter of the screen's port[s], which is pretty close) and even 15' is still too short to be useful for, say, a projector, or a "the A/V receiver or HDMI switch is over in that cabinet, the TV is on this wall across the room" situation.

HDMI goes 25'+, no problem.

displayport has supported HDR10 since 2016

and displayport 2.0, since 2019, has supported all the same variations (hdr10+, dolby vision) that HDMI does

Do you mean in practice, or something? DP definitely supports HDR, and it seems to work fine for me.

Confidently incorrect.

My main monitor is 4K 240 hz HDR and it works great on my DisplayPort cable, especially the HDR.

This seems wrong to me? I use it to do so every day.

If true, not supporting HDR is a feature