Yeah I should have said Android TV since that's what the Nvidia Shield uses. No casting needed and millions of would be Apple TV+ subscribers.
On the other hand I suppose Apple did choose to focus on building an app for the Android TV version v9 built into some recent model TVs and the version of Fire OS based on Android that runs on the Amazon Fire Stick 4k.
I would be very curious if the app on any of these supports Atmos. I know the one on the Fire Stick 4k does not and everything I'm seeing is Apple TV+ app support for Atmos is limited to Apple devices only at the moment.
I have a Sony 930D with Android TV v8 and the 2017 model of the Nvidia Shield TV that runs Android TV v7
The 2019 model Nvidia Shield TV runs v9
Re:
web version access with Atmos support (at least on Windows), that appears to be a no go as well. The only windows browser that supports Atmos is Microsoft Edge and that one is not supported by Apple TV+