My view is that "echo chamber" is a meaningless phrase meant to malign people who you disagree with, but who generally agree with each other.

I don't know what the majority opinion is. I figure it's different for different people, which is why I asked the one who posted this question to gives a more meaningful description.

What is your definition of an echo chamber? It obviously includes "a place where people don't threaten each other with physical violence."

BTW, you wrote "HN are better educated (and therefore more liberal)"

That is a conjecture. Engineers and tend to be more conservative than even the nation as a whole, and HN has many engineers. (See for example https://rationalwiki.org/wiki/Engineers_and_woo .)