As for posts that go against the grain not being deleted - sure hasn’t been what I’ve observed. Usually any ongoing argument/discussion will garner enough reports from the prominent side (ie in line with the views of people who mostly frequent these boards) to be deleted.
And again, posts are only deleted if they do not stay within the guidelines. The guidelines apply to both feminists and activists and those who don't believe they are activists yet use the same tired trope that activists use. Usually, depending on the current political happenings, MNHA might be more lenient to activist posts than other posts.
So, obviously your 'against the grain' posts that you refer to are probably making negative generalisations or are 'against the spirit' of debate. But please, do crack on insisting that well constructed and argued posts will get deleted because someone disagrees. I encourage you to post widely and often to test your theory.
Opinions are like anuses and everyone has one. When it comes to facts and claims that can be proven or disproven, then yes, evidence is often requested or proved by regular posters.
Why do you keep talking about echo chambers? Do you think that you have landed in an echo chamber?