Note that banning "all porn" is easier than accurately sorting child porn from regular porn at that scale as it lets you avoid pissing off petite 20 year olds or getting in trouble because your moderators OKed a report of what turned out to be a more developed 16 year old.

So yes, Apple may only have required Tumblr to more effectively moderate to prevent child porn, but from a business feasibility point of view the practical way to do that was ban all porn.