yes, i think the theory is proved to be working, if you use watch page views* with a very long silent video, you can squeeze some watch time from it, since part of the users will stay on that video until it ends...
https://i.gyazo.com/0e89855886be5bf50240454dd2baddd1.mp4
also it's interesting that we get more watch-time on windows platform
suggested future tests:
1. use RAV views with a 3 hours of silent video - to see if it triple the watch time - this can reduce your overall cost
2. use RAV views with a 3 hours of non-silent video - just to make sure the key is the silence
3. use real ADS to promote a playlist with a few long ass silent videos from your channel
also if all this does help to achieve fast effective watch-time, i'm not sure how YouTube will treat those long silent videos...