The focus of this article is on right-wing extreme content, but I think the tendency is true on both sides. The fact is that partisans are more likely to seek out news content, and partisans are happier with content that matches their prior viewpoint. And so the extreme content rises in popularity and is presented as the recommended option to the less partisan.
It's a worthwhile question as to whether this is a flaw in the algorithm or if the algorithm is simply accurately mirroring democratic preferences. Maybe everyone's tendency is toward more politically extreme content, and youtube is simply giving the people what they want. Which is, after all, what YouTube probably sees as the correct goal.