HDR is about the peak brightness that the screen can reach. The truth is there are HDR TVs with 400nits of brightness but they are not UHD Premium certified - which means that the screen to be certified has to reach at least 1000nits (and be 4K/UHD).
The iMac is up to 500nits - so it could support HDR but I am not sure if that would be a software update or something to be implemented to the hardware either physically (in which can it isn’t) or via firmware.
But even if it did support it the HDR wouldn’t be as good.
However ask yourself this..... Do you want to be sat in front of your iMac watching a film at 500nits!? I would say NO.
Your not sitting next to a TV so its different. (I accept that there are scenarios where maybe you would watch something on the iMac from far away but Apple don’t cater to exceptions to the normal use.)