EDIT: I watched the video. Below the line is my initial post in its entirety. Above it, I'll post my response specifically to the video.
So the theory offered in this video is that Riot sometimes intentionally matches people with "bad" teams, to ensure they have a near 50/50 win rate. Which seems counterproductive, since the easiest way to manage a 50/50 win rate is to get players into the ranking appropriate for their overall skill level (and this system would actually delay or prevent that).
It also seems to be based on a single person's experiences - and a person who has little-to-no knowledge of statistical analysis, either. Or perhaps he's simply never heard of Occam's Razor. Either way, I wouldn't consider this a particularly trustworthy source of information.
I'll edit in a specific comment regarding the video later (I'm at work, and have no sound to hear it with), but I want to go ahead and register my skepticism.
There's no benefit to Riot "forcing" players to lose, or in any other way "enforcing" some arbitrary ranking. Without either solid evidence that they are doing it or an extremely convincing motive, I'm going to have a hard time accepting that idea.