It seems irrelevant whether this person is using encrypted channels if they failed to maintain anonymity. If they distributed material and leaked any identifying info (e.g. IP address), then it would be trivial for investigators or CIs to track them down.
It sounds like he created material, not only AI but actual children then distributed it. The tools to track down the creators of CASM is only getting better.
A single legal image of any of those children posted to social media is going to allow algorithms to make the match and its routine detective work from there.
It only takes one child to talk. No amount of encryption is going to stop that.
In the list of apps he was using I don't see any mention of a VPN. How much you want to bet he raw dogged it with encrypted apps over the clearnet so it was trivial to leak his real IP address
He posted the AI filth to a "public server", so I'm willing to bet his plan was just full of holes. I don't mind pedos getting taken down, buy I do mind encrypted software being owned by the government - any government.
Likely, data may have been encrypted but he may have leaked compromising metadata. Even more likely it was bad operation security linking a personal identity to his anonymous persona.