I have also encountered people who have referred to Boing Boing as a hive of leftism, yet here we are. Also, without knowing more I don’t interpret “a leftist alternative” as saying “only leftist”. There are very few places hospitable to us, so for us it can be an alternative, but inclusive of others who don’t strictly need such an alternative. Not unlike how a queer, feminist, or indigenous rights fora might exist to make a space safe for such people, their discussions and concerns - but not necessarily exclude other good-faith participants. I would have guessed this practice to be sufficiently understood, that I am surprised by the dismissive attitudes here.
Besides, the far-ish left is quite far from being an ideologically homogeneous ecosystem!