An echo chamber fosters confidence in a single perspective, or a small subset of many perspectives. And not everything we speak about is mathematical and can be formalized so that we can have confidence in a single perspective (social, political issues).
Can you, in particular, handle being confident in one perspective and being able to acknowledge, understand, and evaluate another? Maybe. Can most people? I don't think so.
"Having a informative and effective discussion at a high level, and having opinions moved, predicates a high degree of common ground."
We can agree on definitions, but that's not necessarily an echo chamber. An echo chamber, or the negative kind of echo chamber, is when a (nonmathematical, nonformal) perspective or conclusion is reinforced by the entire community.
Example: "everybody has the same opportunities and therefore everybody is equally capable of achieving success". You get people on hacker news asserting this all the time. This is not a definition. This is dogma.
"Being bombarded with information and viewpoints you consider wrongheaded doesn't expand your horizon, it galvanises you."
Key phrase: "[ideas] you consider wrongheaded". Nobody has all the right ideas from the start. A priori, there is no reason to think that you will never encounter any ideas which appear wrongheaded to you at first.