I don't know if you're right or not, but certainly YouTube and desktop iTunes is only a subset of AirPlay content. This doesn't really scale without providing a spec for content to follow.
All iOS apps must use HLS for videos longer than 10 minutes and Apple verifies compliance during the app store review process. As mentioned, with AirPlay all HLS media files are downloaded directly by the AppleTV, not piped through your device. I can tell you from experience it works very well.
1) Found a video on iPad YouTube. Longer than 10 minutes. 2) Played it to the TV with AirPlay. 3) Turned the iPad off.
Expected the video to continue to play but it stopped immediately.
Can you get a similar test to work?