Post #2 is correct in that your TV not supporting ATMOS has no effect on being able to enjoy ATMOS sound. Why? Because your plan is basically redirecting the audio signal before it even gets to the TV: AppleTV audio to HPs and video to TV. The TV has- in effect- no say in the matter.
Real ATMOS involves many speakers spread around the seating area, including some overhead. You can't get it with HPs or any other pair or single speakers (regardless of what it says on a box or in the marketing).
If you want the real thing, think ATMOS receiver and at least a surround sound setup (real surround sound, not the faux kind pitched on soundbar or stereo speakers boxes/marketing). If you are able, get some speakers overhead too. There are no movie theaters in the world with only a HP or two down front. Go watch any movie and before the lights go down, take a good look around. You'll see physical speakers down front, to your sides, behind you and probably overhead. Why do theaters spend all that money if a couple of relatively cheap HPs can deliver ATMOS? (rhetorical- the answer should be obvious).
As also offered in post #2: HPs sound great. But they are stereo when paired... with no current- nor rumored- way to make them more than that. They will sound farrrrrrrrrrr better than the TVs built-in speakers but can't compete with a good surround sound setup (anyone's ears should be able to tell the difference). I think of HPs as best at their original intent: speakers for music, especially in places where it may not be easy to work in traditional stereo and speaker setups.
This "make them home theater speakers" (too) push is Apple fans striving for something that is fundamentally limited from the start. If Apple really intended to get into THAT business, there would be a center speaker HP, a HP subwoofer, surround HPs, etc. Apple likes money. That would sell many more HP speakers. But Apple has shown no interest... nor are there even ANY rumors to imply "maybe someday."
Real surround sound has been in home theaters since the early 1990s. If you want it, you can have the real thing. It won't even have to cost much more than a single good Mac or iPhone PRO purchase for a great, basic setup. Unlike that Mac or iPhone, speakers typically last 20+ years, sounding as good in year 20 as they do in year 1. You might spend 2X-3X or more on Mac or iPhone after Mac or iPhone (after Mac or iPhone) in that same timetable.
The point: consider putting some Apple-like money on this LONG USEFUL LIFE purchase you will enjoy for probably a few decades. Current gen HPs will probably be made software obsolete within only a few years (note how Gen 2 can't even stereo pair with Gen 1 from just a few years ago).
However, audio is an "ear of the beholder" thing. If HP stereo (and heavy walled garden "lockdown") makes you happy, enjoy them.