I think you're missing the point. What makes this thread an echo chamber is not that dissenting opinion is ignored or shut down, but that it is clearly viewed as an unwanted inconvenience; a fact made obvious by the absurd degree of acrimony even the slightest disagreement invariably attracts here. Perhaps this doesn't meet the strictness of your definition, but in my book, if people would rather just have their own beliefs reinforced, then they're partaking in an echo chamber, and the fact that even the mere suggestion that this is the case can provoke such a reaction here should be entirely illustrative of my point.
Personally, I find this point of contention to be largely tedious, and would rather discuss something of more substance, such as the validity of the current claims going around about voter fraud, or Trump's various statements on the subject. I've been trying to find some hard evidence to back up the claims, but so far all I've come across is hearsay. Would you happen to know of anything more concrete?