Let us argue the point including the "under 18" part, then. If it was legally an issue I can understand the ban. Otherwise I see little justification in the decision.

Actually it banned anime porn that depicts characters that look under the age of 18, that excludes characters like Oshino Shinobu and most Touhou characters for example. Only a very small minority of anime porn portraits characters that are and look over 18. Saying that Reddit banned anime porn is as correct as saying that a byte is 8 bits: while wrong in theory it is basically true for the vast majority of the CPUs.