Many filters censor all of it, regardless of legality and boast 99% abuse detection rates with a low false positive rate. Some filters only the more realistic content.
Realistic content does admittedly have a problem where it may become difficult for a human to differentiate it from unlawful content. Although in that particular case, it is hoped it would end up competing with unlawful content and drive demand for it down.
People with some degree of awareness to the children harmed to produce the content will not want to contribute to it when there are other avenues which are virtually indistinguishable. It may be possible to do outreach in legal outlets to raise awareness of harms or provide resources to prevent them from committing serious crime.