The author explains himself here: https://lemmy.world/post/49817942.

Like apparently it's not "leftist" per se (or trans or furry etc.) so much as extremist rhetoric, or even instances turning a blind eye towards same, for the sake of mainstream normies coming here from Reddit.

It would be like blocking pretentious people, then vegetarians decrying unfair/biased treatment.

Note that many leftists are not blocked, like I strongly doubt that slrpnk.net would be, it's primarily the more "militant" varieties.