According to the documentation I think it does mean what he thinks it means. Apple is adding a new option to the audio output to allow for passthrough audio via the HDMI (the AVAudioContentSource now has a new function called passthrough. This function tells the Apple TV which type of audio content is coming into the device and up until xOS26, there was no passthrough option, which according to the documentation tells the xOS device to pass the audio out unprocessed via the output. I say xOS because this variable is present in all OS26 versions, not just tvOS, which has huge benefits to iOS and ipadOS too). Whether or not Plex will add support for it though is a matter of time to see.
With regards to your second point though, that one is a bit different. The reason why Plex apps do not support that is because Atmos support in the API today is only available via certain audio containers. Atmos can presently be supported if the audio container is in a specific compressed format, which is really only used by streaming services like Netflix due to needing to be conservative on bandwidth. Atmos isn’t supported in any of the uncompressed containers (such as Dolby TrueHD). By providing this new function (and if Plex adopts it), we may finally be able to have the one thing on Shield that isn’t present on the Apple TV today in terms of audio and video output.
