The only manufacturer offering dedicated HGiG mode on TVs is LG (AFAIK) and this mode doesn't read anything from the source. It's just basically turning off tone-mapping, follows the EOTF curve until it reaches TV's peak brightness and then hard clips anything above.
So new Sony TVs also have an option to turn off tone-mapping with a hard clip, it would be equal in effect to LG's HGiG mode but the difference is that Sony undertracks EOTF curve meaning everything is darker than it should. All they need to do is correct that with fw update.
Correct, it is not DTM. The display is reading the tolerances for peak brightness from the console. When we say a game "supports" HGiG, we're really just saying the game uses the console to determine calibration, which by extension uses the television (what you visualize as clipping or black crush) to determine. It's just been a terrible occurrence of nomenclature. The HDR world is still without standards, other than DV and HLG.
My question for Sony televisions would be in relation to how the console communicates this to the display. It may have a non-tone mapping/anti-hard clip option, but is this just a guessing game? I'm assuming this is the case, which is why I now see them getting ready to implement some kind of official HGiG solution.
But again, it just continues to baffle me why Sony is a laggard in this space.