I think this is a good observation, but I think the diagnosis is slightly wrong. I don't think the process of becoming dismissive of opinions different to yours is limited to those with "extremist" views.

The people who agree with you are a biased sample of the population. But so are the people who are willing to openly disagree. So we have both a "bubble" and an "anti-bubble". This is especially true for people with unusual views, but it's also true for everyone.

I had a colleague who was part of an idiosyncratic religious denomination, which believes in the literal truth of the Bible etc. He was a nice guy and mostly we avoided talking about this. The one person who raised it in social conversation was the most tactless guy in the office. (To be fair he wasn't that tactless. Someone has to be last). Lots of people online are looking for a verbal fight. The decent people who know you socially or via work, but disagree with you over some political or religious issue, will usually avoid talking about it with you. That applies just as much if you have a mainstream but not universally held belief, as if you have an extreme one.

The problem here is that there's not really any incentive to seriously engage with a person with opposing views, even though our society would be a lot better if people did it. Lots of people seem to think that freedom of speech just means freedom to harangue those you disagree with, and aren't interested in the search for truth.