I found an explanation in the topic Tablo iOS Mobile App (4th Gen) Update (v. 1.4.2 & 1.4.3) - #11 by TabloTV, where there’s a post from Tablo:
"Unfortunately this is another symptom of Apple devices not handling playback of MPEG2 video. The AirPlay button should be disabled when you’re playing back live antenna TV or completed antenna TV recordings when you’re using expanded storage as this would result in the issue where it would cast the audio but not the video.
Free streaming channels and completed recordings on onboard storage that have had time to be archived as MPEG4 video should offer the AirPlay option and should work as intended."
This makes me wonder whether the app for Apple TV, when it’s released, will even be able to play live TV or recordings on external storage.
Can someone from Tablo respond to this? I submitted a question to support, but got the following unhelpful reply from an automated assistant saying:
“The 4th Generation Tablo DVRs can decode MPEG-4 broadcasts thanks to a firmware update. This means that you should be able to play live TV on your Apple TV 4K using the 4th-gen iPhone app, provided the channels are broadcasting in MPEG-4 format.”
Clearly, my problem is not MPEG4, but MPEG2 video. I’m hoping that a human will eventually reply with a better explanation of MPEG2 on Apple TV devices, because if the Apple TV app will never be able to handle MPEG-2 video, then the entire 4th-gen ecosystem will be problematic for Apple TV users.