In an echo chamber bad posts from one side stays up and gets upvoted, while posts from the other side gets extremely high standards. I don't see that on HN, but a lot of people who are used to being in an echo chamber can think seeing bad posts from their side getting downvoted means this is an echo chamber, when it is the other way around.
If you say the only answer is a revolution, or if you say any potential solution is moving too fast, and we need to take way more time to consider the situation, you probably get less support regardless of which side of the traditional left/right divide you're championing.
I cannot speak for MetaFilter, but Wikipedia has a long-standing history of denying factual information in favor of the “accepted view”, particularly on social issues, and uses its “only accepted sources” to abstract this bias away (despite the double standard application of said rule).
I see both conservative and progressive points get upvoted to the top in different threads, that is strong evidence that this isn't an echo chamber. On r/politics I never see right wing opinions get upvoted to the top, and on r/conservative I never see progressive opinions get upvoted to the top, that suggests those two are echo chambers unlike here.