Again, dig back into the threads from last years tvOS. It was one of only about 3 or 4 bulleted features of the new tvOS 15 at the time... that was pulled at almost the last minute for no obvious reason. I couldn't quickly find the simple bullet list tvOS 15 roundup from last year but
here's an interesting read rounding up the prior generation Apple TV being ready for BOTH HDR and DV HDR in the wayback machine. If you worked forward in time in the wayback, you'll eventually find multiple threads talking about it then, expecting it, it being in a short list of key features, then suddenly pulled.
One year later, there is new hardware and it is one of a handful of tangible benefits limited to only the new model.
Yes, I admit I can't recall with certainty if any of that was HDR10+ or not. It may have only been HDR10... but that would still imply HDR10 could be delivered in tvOS 16 for those sufficiently pleased with that and owners of TVs that work with it... including many that do NOT work with DV.
Consider too that Dolby Vision isn't a significantly less taxing implementation of HDR support- just a
different version. The older hardware has no problem with Dolby Vision support. HDR10 isn't some "next level" HDR... just one that is used in place of paying the DV license fee on the most-sold television brand in the world and a few smaller brands support it too.
My gut guess is that HDR10 would run just fine on the prior generation AppleTV... but Apple needed some exclusives to help push this new one and it become one of them. Anyone with the most-sold TV brand and interested in HDR will likely want to upgrade to the newest AppleTV to use that feature (that group includes me). if their existing AppleTV supported it, they could just update tvOS and carry on.
See features like you describe with that iPhone 3GS limitation and plenty with macOS too. It works really well to accelerate the pace of "upgrade" purchases... just like mysterious, "long in tooth" slowdowns with each iteration of what should be increasingly-more-refined OS versions.