I'm really disappointed with Apple! According to Dolby (link below) sending DD+ and Atmos over PCM is substandard. I suspect Apple has chosen the quickest route to support Atmos with ARC along with DD+ but not TrueHD. Due to bandwidth constraints, it's not possible to support TrueHD and Atmos with the original ARC together and this is why I suspect TvOS only has native support for DD+.
https://developer.dolby.com/blog/dolby-audio-over-hdmi-part-2-signaling-and-carriage/
Apple really should just give us the option of bitstreaming the audio to get TrueHD and Atmos (thus losing the ARC capability) to our AVRs and TVs. The second preferential longterm solution would be to support eARC which operates at a higher bitrate to support ARC and Atmos.
If Apple were to allow bitstreaming audio then this would likely mean Siri would no longer work because Apple would not be able to include system sound in the DD+ stream but I would to sacrifice Siri and system sounds during playback to get proper Atmos along with TrueHD through my home theatre receiver and 7.1.2 system.
It really disappoints me that a company the size of Apple can't put more resources onto the ATV 4K to make it a killer device!This is 2018 and DD+ was the standard of 10 years ago. My 2017 AVR and 2018 OLED supports eARC which allows Atmos along with ARC. Why can't the 2017 AppleTV?
There are Android devices available for half the price that offer bitstream Atmos and TrueHD output over ARC. Unless Apple can offer this option then we have to look to other devices with proper media streaming functionality or wait forever for the AppleTV to support eARC and knowing Apple this will likely require purchasing a new Apple TV4K (version 2?)