@StreamerUser wrote: Its DD or DD+ stereo-only (2.0 channels), thus "DD2.0" or "DD+2.0" as opposed to say "DD5.1" or "DD+5.1" - this is common in TVs (E.g. LG) that have built-in DD+ decoders/encoders/transcoders and built-in apps - instead of outputting PCM 2.0 they can/do/sometimes output DD/DD+2.0 (depending on the detected ARC/eARC device and app) instead.
A little reading on the subject...
What distinguished DD 2.0 from just regular PCM stereo (other than file compression) is that with DD 2.0, you could possibly get matrixed audio on those two channels. In other words, 4 channels of audio could be decoded out of two discrete channels of data. This is what Dolby Pro Logic (and its later variations) provided - a rudimentary surround experience. Dolby Digital, Dolby Digital Plus, et al., provide actual separate discrete channels of audio.
Here's how a Dolby Surround book defines 2.0:
"Dolby’s ability to encode and decode information is only as strong as its source data. If it’s working with a two-channel stereo movie, you may see something like “Dolby Digital 2.0” on the package, designating that it is a stereo signal being encoded and decoded using Dolby Digital. All fine and dandy, but it’s still a stereo signal. However, if your receiver includes Dolby Pro Logic, Dolby Pro Logic II, or even Dolby Pro Logic IIx, the stereo signal on the disc can still be listened to as a multichannel surround signal, thereby allowing you to achieve full playback on your system."
The idea of the poster being that at the time of upload to YT the 5.1 audio is surround matrix encoded (Lt/Rt) in 2.0 and then transcoded to AAC-LC (Stereo+5.1) (which is what YT uses) and then, at playback, depending on the device/app capabilities (e.g. Roku 4640), the AAC-LC (Stereo+5.1) encoded is then transcoded back to DD5.1 (or DD2.0) or DD+5.1 (or DD+2.0) with the Dolby decoder/encoder in the 4640 for output via Optical/HDMI.
(This is what my LG TV does in fact with (some of) the internal apps - 2.0 AAC/PCM is transcoded to DD2.0/DD+2.0 when output.)