That's explained in the paper:
> Our list of extremist channels consists of those labeled as white identitarian by Ledwich and Zaitsev (26) (30 channels), white supremacist by Charles (45) (23 channels), alt-right by Ribeiro et al. (24) (37 channels), extremist or hateful by the Center on Extremism at the Anti-Defamation League (16 channels), and those compiled by journalist Aaron Sankin from lists curated by the Southern Poverty Law Center, the Canadian Anti-Hate Network, the Counter Extremism Project, and the white supremacist website Stormfront (157 channels) (46).
If you're claiming to fight extremism you should also fight extremists that support your political views.