That's a very selective reading of the article. Twitter boosts things that attract more views and shares. Outrage generates those views. But why would mainstream right-wing politicians and articles generate more outrage on Twitter than non-rightwing content?
That only makes sense if Twitter is predominantly anti-right wing, and people who are anti-rightwing get outraged by rightwing nonsense more than leftwing nonsense, and thus rage-click, share and comment on that content more.
There's another theory that fits the data: rightwing sources are more likely to share things that are enraging to rightwing readers. After all, rightwing sources are mostly followed by rightwing readers, same as with leftwing. Usually they just stay in their bubble. As to why rightwing sources would do this, I can think of a couple of reasons, but they are outside of the scope of my comment.
That only makes sense if Twitter is predominantly anti-right wing, and people who are anti-rightwing get outraged by rightwing nonsense more than leftwing nonsense, and thus rage-click, share and comment on that content more.