He's using the word "criminal" where I think he should have used "adversarial". So what stops an adversary from embedding CSAM in the Bitcoin blockchain? Nothing really. They could do this since day one and since many years with a 100kB OP_RETURN. Coming policy changes did not affect that one bit. On the other hand, the claim that some CSAM sharing group is using Bitcoin for their stuff is totally ridiculous and transparently just an attack against Bitcoin. Can you put an occasional CSAM on the blockchain? Sure! Can you use it for an unstoppable file sharing protocol? No! And if illegality was the issue: There are many OFAC sanctioned transactions on the blockchain already and nobody, not Core, not Knots intends to block them.