Well, that kind of tanks your whole argument, doesn't it?

A fairly prominent, completely non-hateful position that is contrary to the dominant thought on the farther left gets zero exposure.

What could be better documentation of an echo chamber than that? It might be self-selected (e.g. you might not get banned if you argue against using the lens of identity as your primary tool for identifying and marshaling opposition to injustice), but if it doesn't come up, it's pretty darn echoey.

(Since this is based on your own perception, we can only tell that you appear to be in an echo chamber, and it might be due to limited sampling on your part--you don't specify just how much content you've consumed there.)