Simply banning adult content en masse allows them to use heuristics to identify nudtiy and genitalia in images.
Previously, they would have had to make a judgment call on whether those depicted were of adult age. By banning all of it, they no longer have the responsibility to make that choice.
That wasn’t the case. They had the industry standard tools (PhotoDNA + partnership with NCMEC), but I can’t say how well they were being maintained at the time of the situation with Apple.
There was some great snark about Safari being next on the ban list.