There are many ways to avoid encountering illegal pornographic content on Mastodon. Indeed the one you linked to does just that, by having a mod staff that can block offending accounts and suspend interactions with instances.
I can't think of any other major platform where avoiding illegal pornography is something users explicitly have to think about and take actions to protect themselves.
I find it worrying that this is even an issue, but then again it's not surprising - if you build a platform explicitly against witch hunts, you'll end up with 1% principled legitimate users that use it to make a statement and 99% witches.
Actually, even Twitter has a CP problem. I've seen it discussed before, where various accounts with CP were reported, but action on said accounts was delayed due to unknown issues on the side of Twitter. Meanwhile, the moderation on Mastodon instances varies from being more lax than Twitter to being more strict (IIRC that instance you linked is slightly more strict than Twitter).
Also, Mastodon wasn't built "explicitly against witch hunts". A lot of people go there because they don't like the hellscape that is Twitter, or because they simply want a different audience. Heck, there are many people --- myself included --- with both Twitter and Mastodon handles. The people who go to Mastodon after a Twitter ban usually end up on more toxic fringe instances, like radical.town (tankie) or social.quodverum.com (MAGA).
Depending on which part of Mastodon you are on, you can still find witch hunts, usually on the fringier instances.
