I'm not defending the FBI or the Apple feature here, but on a lot of cases where abusers get caught it's because they were sharing pictures of their victims in groups where they exchange other pictures with other people into that. Sometimes those other pictures are on this databases. So this database matching things generate leads.
Last case I heard about was cleaning personnel in a body expression workshop for kids with learning disabilities that was sharing new pictures he took of girls in a Telegram group. The group was infiltrated by an FBI agent that allegedly got the link from a Facebook group that they found because of Facebook scanning for known hashes and reporting.
I can't remember the actual title of the article but I clearly remember about an instance (I think it was a few years ago) where there was a CP ring that operated (at least partly) through a whatsapp group and they got busted when they accidentally added someone with the wrong phone number so yeah, I think there's a lot of "low hanging fruit" that could lead to putting some of these assholes behind bars.
Last case I heard about was cleaning personnel in a body expression workshop for kids with learning disabilities that was sharing new pictures he took of girls in a Telegram group. The group was infiltrated by an FBI agent that allegedly got the link from a Facebook group that they found because of Facebook scanning for known hashes and reporting.