We might be talking about different things.Isn't purchased iTunes video content physically downloaded to your computer? I've only bought one thing on iTunes, an episode of Frontline, and it's physically downloaded and stored on my computer. But that's my only venture into buying from iTunes.
I thought you meant HTPC ability, running eg Plex to play Atmos content from your server.
I no longer know the ins and outs of how AppleTV plays iTunes content. I assume UHD Atmos content is streamed. No longer downloaded to a desktop and played across your lan. But?