That's the BS reply that always comes when (in particular) bitcoin is associated with crime: that it is traceable. Well, it hardly is. And for the average policeman/woman, even less so. And when the owner has taken some care, as you admit, practically impossible even for experts.
It's not BS though? That's what companies like Chainalysis professionally. This was covered in graphic detail as to how they caught a pedo ring in Andy Greenberg's book Tracers In The Dark.