[cocoricamo]

Looks to me that right from the start the study has a fatal flaw and it's clearly biased.

Just looking at their channel selection is clear that they consider extremist only some forms extremism. Particularly those often considered as aligned with right wing politics.

From the Ledwich and Zaitsev paper they reference only the "white identitarian", "anti-sjw" and "men rights activism" categories are selected. But there are more categories that could be considered extremist at a glance like "revolutionary" and "anti-whiteness".

Other categories in Ledwich and Zaitsev's paper: Conspiracy, Libertarian, Anti-SJW, Social Justice, White Identitarian, Partisan Left, Partisan Right, Anti-theist, Religious Conservative, Socialist (Anti-Capitalist), Revolutionary, Provocateur, MRA (Mens Rights Activist), Missing Link Media, State Funded Channels, Anti-Whiteness. Some channels in other categories could be considered extremist but would require a nuanced analysis.

About the other lists or what actual channels they looked into I can't really say because I can't seem to find the actual list they used. If someone has the full list and categories they used please link it.

While it could be possible to analyze the behavior of extremism using only right leaning extremism, it is possible that this pattern does not apply to other forms or ideologies.