According to the release notes for Infuse 5.8.2 tvOS, HLG video is supported on the Apple TV 4K. However, I’ve been unable to get HLG to trigger with any HLG files (sourced from iPlayer). Apple doesn’t mention HLG anywhere officially but I remember references to HLG turning up in early tvOS .plist files back in 2017, and we know that iPhone 12 records video in HLG as well.
HLG triggers fine using Plex on my Roku stick and the new Google TV dongle, so it’s not a content or TV/AVR issue.