It isn't. "Hugbox" is used pejoratively specifically to attack people with perceived left wing beliefs and associations. The idea is that people who aren't interested in seeing a ton of transphobic content or whatever are too "soft" to expose themselves to "real conversation."
"Echo chamber" is more neutral in orientation, though still carries a negative connotation. "Hugbox" is specifically slanted.
As for not wanting to see transphobic content, I think that's entirely a reasonable position. People shouldn't tolerate abuse. Abuse != "real conversation".
People are weird.
And even apart from that, I think everybody has a right to choose who they associate with, and that does not have to include harassers. That's more relevant to social media and less to teaching classes of course, which do need to be open to everybody. But if you're choosing to follow a class with the specific purpose to harass someone, there's something deeply wrong with you.