I’d say CSAM distributors would be just as interested in the workings of CSAM filters as email spammers are interested in how spam filters work, so there are reasons to keep these systems under wrap.

One answer is to draw the line far away from where it strictly has to be. If you ban all explicit photos and videos than you don’t have to split hairs to detect CSAM (e.g. was that person 17 years and 364 days old or 18 years and 2 days old?)

Sites that host many lewd anime images tend to reject cosplay and other real-life images, sexual or not. People will accuse the anime industry of producing images that sexuality young people but if you keep any actual people out of it you can’t draw a line between a particular image and an actual abuse event.