I don't know why anyone is surprised. The edgy subreddits (many of which were toxic cesspools) all warned everyone as they got banned, but people cheered as it happened. Finally the baddies were gone. It was never going to be limited to just them.
On another note, I do not understand why all these banned subreddits that try to migrate to their own site always try to create their own software. the_donald did it famously and created an absolute clusterfuck of a website (and community). There are so many options, there's open source old reddit, there's federating varieties like Lemmy and littr.me (a good choice if you want to run a single community), why reinvent the wheel? If you want to move to an environment like reddit where everyone can interact, but give community moderators full control over their communities, a federated architecture is the way to go, and there are a couple of solutions that work out of the box.
Not all. The admins of Ovarit [1] decided to use Throat [2] rather than roll their own from scratch. Their fork [3] has had various customizations added over time, but it largely follows the upstream. By all accounts, this arrangement is working very well for them.