Before anyone start making their own conclusions, one must understand how HDMI signal transfer works and how apple has done it.
I dont
For apple to "convert" and make some sort of "fake" HDR with a SDR fource is possible..
But I would say that is more likely that the SDR signal/video is sent unchanged within the HDR stream. After that is the TVs own "HDR" proccessing (Because you are i fact on a HDR picture mode on the TV) that would tweak the SDR pic further.
I have played a few SDR movies to test and they look very much unchanged, as my TV's HDR mode will (LG B6 mode DV Dark) have a little bit different settings than my normal "Cinema" mode of course the pic will look different. HDR modes on an OLED put brightness/OLED Light to max by default.
Also on a new source (ATV) the default mode is too vivid and with a lot of post processing, dynamic contrast, edge enhancement and on on that one would have to turn off.