You're right, the population of people under suspicion is not an iid sample of the overall population and it would be unreasonable to expect the same percentage of abuse. However, we choose who is in this population by defining the set of rules for which cases are treated as legitimate and which are dismissed. If it skews in the wrong direction, that's a failure of the system to properly filter cases.

Yes a kid who ran into a doorknob is more likely to show up at school with a visible black eye than the kid whose mom is used to covering black eyes with makeup, but CPS knows, or at least should know this and alter their behavior to spend less time investigating a single unconcealed black eye and spend more time looking at the kid wearing concealer on one eye on a regular basis. The whole point of having trained professionals investigate is because we know that accusations are a noisy signal that must be filtered.

I fully agree that the 16% number should not be treated as a target. For starters, that's just sexual abuse, the rate of physical or sexual abuse is necessarily higher. But the point is that when evaluating a system you can't simply say you find abuse at a non-zero rate and thus it's working - a broken clock is right twice a day, and doing anything at all will find something. You have to compare to a placebo to see what effect your actions are having. If you're substantially worse than the placebo, that doesn't mean sugar pills are the cure, but it means you need to do things differently.