"Basically if there is an internet community that's not expressily rightwing, it's community will be leftwing, so leftwing people don't need a space for themselves"

I'd argue the exact opposite. Most of the big internet forums, reddit, 4chan, gaming forums are all right leaning to greater or lesser degrees, and most of the smaller forums I've come across in many many years of wasted time on the internet follow this pattern. The lack of a left wing forum is perhaps more due to the demographics of internet forums in general and the fact that forums as a medium are inherently a little abrasive and open to trolling.