My only guess is that they’re one of the “AO3 is for pedos” people. There is a vocal group that takes issue with AO3’s lack of content-based moderation. AO3’s stance has basically been that they’ll only censor things that are directly harmful to people. And by extension, they maintain that imagined content cannot be harmful to real people, so it shouldn’t be censored. Basically, following that line of reasoning, images of CSAM would be prohibited (not just because it is illegal, but also because a real person was harmed to create it) but a post describing imagined CSAM would be allowed. And this is what lots of people take issue with.

Basically, AO3 takes a “the audience is a genius” stance to things, where the audience is what decides what is and is not too far. The term was initially coined to describe comedy, where the audience is the ultimate judge of whether a comedian’s joke is “too far”. Even if the comedian doesn’t believe it was too far, the audience has the final say because they’re the ones who decide whether or not it’s acceptable to laugh. If a comedian tells a dirty joke and nobody laughs, the comedian instantly knows that it was too dirty and they need to dial it back.

The stance can be applied to other topics as well though. In AO3’s case, it essentially means that individual moderators shouldn’t have the authority to block content they find offensive, because what may be offensive to a mod may not be offensive to the audience it was directed at. The concept of a genius audience is that the audience is always the final arbiter of what is and is not acceptable. That the boundary of acceptableness is a push and pull game between the content creator and the audience, with the content creator essentially toeing the line and seeing how the audience reacts. If they react positively, the creator knows it’s not too far. If they react negatively, the writer knows they crossed a boundary.

To bring it back to the original topic, some people have a big issue with the fact that AO3 uses the genius audience stance for moderation, because they maintain that it enables pedophiles. Basically, that the stance is fundamentally flawed because there will always be another pedo who reacts positively to the content, but that doesn’t mean the content is acceptable. In practice, a lot of the times it is applied to ships that involve canonically underage characters. Maybe a series gets popular, but all of the characters are only 14 or 15 years old. That won’t stop ships from popping up, but it will trigger accusations of pedophilia when people post stories with those ships.

To use an older example, lots of people had issues with any ships involving Avatar: The Last Airbender. All of the main characters were like 12 or 13 when the series started, and had only aged (at most) a single year by the time the series ended. It also had an absolutely rabid fan base. So ships about the characters inevitably had at least a few “eww these characters are literal kids, you shouldn’t be shipping them” critics. But AO3 refused to police the ships (or any stories involving them) because it’s fictional content.