What’s extreme about them? Does being outside the American Overton Window automatically make one an extremist?

No, they're not. The subreddit was, at the same time as the Trump subreddit, in a transparent attempt to do a bothsidesism and limit PR damage to . The cited extremist content were related to John Brown being justified, etc...

Generally the subreddit was to the left of Chapo, and there was a signficant amount of distance between the two, with the sub and the podcasters roasting each other every so often.

FWIW I originally learnt about Chapo Trap House on this website, a year and half ago or so. They're obviously a podcast so not as accurate as written publications, but as long as you take their bias into account they're not so bad.

To substanitate the podcast not being banned, check out r/blackwolffeed which discusses the podcast (unlike the eponymous subreddit), and you will see that it is in fact not banned from Reddit.