The simplest view is that these safe spaces mostly exist because some larger platforms have been turned into safe spaces. However, even on larger platforms, these types form insular groups that are effectively walled gardens within a larger ecosystem (see T_D). The mode of operation appears to be to rally behind safe spaces, then brigade larger platforms when something catches their attention. No real discussion, just bombarding text.
It's exceedingly difficult to have honest discussion with those of opposing views because people reward themselves with safe spaces and tend to expect them. Ideally, we could rely on rational discourse to assuage the extremism issue and inoculate. If we shout them down in a given arena, they'll scurry away but never disappear.
As I see it, it's a problem of scale. On smaller vbulletin forums you'll more likely see heated, but honest, debate there without any brigades nor bans (depending on modship). A rainbow of opinions. With platforms as large as reddit, censorship is holding all out shit shows at bay.
Liberals have nearly the entire internet as a safe-space. It's nice to have a community with similar views, especially political as people are always just looking for confirmation bias, not really willing to debate, so it's too toxic to try all the time.
That's not what a brigade is.
> To my knowledge there was no coordination of all users to brigade.
I was in the Discord and an active member during the 2016 election. It was actually really civil, more-so than other political subs. Outsiders aren't as welcome as supporters, but try to be a Trump supporter in the /r/politics sub.
tl;dr There was no brigading by T_D, Reddit just didn't like conservatives using their site. Post proof if you have it, otherwise I'll go with my experience with them.