Why reasonable? Don't these ads pass through pre-publication review, and Meta makes money off of it? Any number above zero is a serious problem. If they can't run an ad business that doesn't publish and profit off of CSAM, they shouldn't run it.
I don't think we should distinguish between 'fake' and real CSAM based on if kids are hurt; this ad machine shouldn't be able to publish it, regardless. The moral question of if it's real or kids were directly harmed seems separate to me.