I recognize that it's "No True Scotsman"-ing but I have a hard time thinking of those "purity police" as "extreme left"
The "extreme left" people I think of are starting co-ops, planning rescues and labor actions, doing jail support, planting gardens, and fighting against the local fash, sometimes quite literally putting their bodies on the line. Nobody committed to that kind of work has time for purity tests & patronizing; they need all the help they can get