I think that stretches the definition of 'echo chamber'. That's simply what a community is, a group of like-minded people with shared views.
There's nothing automatically wrong with this. People can have shared criticisms of others if there is some meaningful difference between the groups that justifies that disagreement. I think people mistake 'hurling outrage' with simply two (or more) groups being fundamentally in disagreement.
The answer to this is actually the opposite of what people commonly suggest. Just let people with irreconcilable differences go their own way which is why I like the researchers suggestion to improve tools to shield oneself from hate.
Instead of demonizing so called 'echo chambers' I think we should just call it what it is, freedom of association.