I posted an answer yesterday, which may still be pending. In any case, I want to add to it. I acquired a new 2021 4K Apple TV yesterday, and it does not appear to support HLG. My test case is the Digital Concert Hall (app from the Berliner Philharmoniker). When I play one of its HLG-encoded programs, the HLG beacon does not light up on my TV, and the picture quality is roughly equivalent to SDR. The same program displays brilliantly, with proper grading, and bright whites without washing out, on a Panasonic Blu-ray player.
I feel it necessary to clarify what I mean by "does not support," because it could be interpreted to mean that HLG programs don't play at all, or that they show as un-decoded HLG. Apple TV is doing something to provide an acceptable picture from an HLG source, but as I said above, the result looks like SDR. As you may be aware, Apple TV's output is set to either HDR or DolbyVision, and its menus and other original material do indeed light up either the HDR or DolbyVision beacon on the TV to which it is attached, depending on which has been chosen in the Apple TV settings (I am using the high-speed HDMI cable ATV requires for Dolby Vision, so that's where I have it). Now, what happens when ATV encounters an HLG signal? One possibility would be that it would process the signal and put it out as, in my case, Dolby Vision; but if that were the case, my LG TV settings would respond to that and provide me with a set of Dolby Vision modes ("Standard," "Cinema," "Vivid," etc.) Instead, it presents me with its corresponding collection of SDR modes. This implies--almost guarantees--that when confronted with HLG material, ATV switches off the Dolby VIsion or standard HDR signal and substitutes SDR.
My conclusion is that ATV "supports" HLG in the sense that it produces an acceptable SDR signal from it, but most definitely not HDR in any sense.
