I think this study is testing the wrong hypothesis. Their idea is that people get "radicalized" by being recommended extremist content after watching mainstream news videos. ie, fox news -> 911 conspiracy videos. They demonstrate that this doesn't happen often. But the real problem wirh youtube is intra category recommendations. If someone watches one conspiracy video, then their recommendations become all conspiracy…
Also it's kind of a weird take that extremism == far right. It seems like far left would be included, again as a matter of definition.