That seems far fetched to be honest
Reuters has been running a series of articles on Onlyfans.
https://www.reuters.com/investigates/special-report/onlyfans-sex-children-accounts/This is only a guess, but there may be an organized group placing the copy and paste accounts. In addition, if they are checked and found to not be illegal, "suspect" but legal spam postings were a tactic that was used in what it seemed were efforts to cripple adult dating websites in the past (though those were mostly spam floods and more innocuous in content likely to make the site unusable rather than especially publically abhorred). This is because despite best intentions it's not really possible to determine age accurately by eye when shown a photograph of a young looking woman. There is a legal review article in some periodical somewhere that ran a test with experts in this field that showed it wasn't possible to accurately place the age of young women by eye by a photograph; I don't recall if they tested with video as well but I guess that would have made sense. I think it can be found on Lexis with some searching.
This website checks government ID for performers so it would be more difficult to spam I would think. I guess someone could use another person's but then it seems like there is still a direct line back to that person, so it would have to be stolen or fake to be difficult to follow? That seems like a broader ID issue if that's what it is. It's also possible that what is detailed in this article of the series about copy pastes could be honeypots that were inadvertently interrupted. Reuters did not receive more information after they were closed.
The article also is missing context at one point. Reuters had received a response from Onlyfans saying the NCMEC had full access behind the paywall curtain. However, Reuters said that NCMEC was limited to tips and ongoing investigations, and did not do basically an active search.
I think the reason this is missing context is because the United States has conflicting federal circuit decisions on the matter of Agency status, or it did last I read about agency law. To sum it up quickly, the NCMEC is heavily funded by the US government. This raises the question of whether NCMEC, a 501(c) organization, is an agent of the US government. This carries a larger burden as far as things such as the 4th amendment are concerned and generally would make certain things more difficult.
The reason why this is the missing context is it's likely not Onlyfan's fault that NCMEC cannot perform searches. NCMEC's statement suggests they are trying to stay close to the boundaries set for not being found to be a US government agency under Agency law, in that it can't (in probably too simple terms) perform a service (such as a search) at the request of the government or else it may be an agent which is also supported by the government funding. In addition if it were found to be in an agency status, it would be subject to the 4th amendment which law enforcement already is and which may interfere to some level with some of NCMEC's core functions. Whether NCMEC is attempting to satisfy both of the split circuit decisions I can't try to hazard a guess from the article, but weighing 4th amendment could also add to being unable to perform (or possibly participate in private actors' process in certain ways for) searches (without a warrant) to some degree if it's duties conflict in some way.
IIRC the controlling precedent on the matter of status as agency at the time I read about it involved a case about Amtrak and it's status, as it too is federally funded.
The article also seems to run the risk of lionizing action against small of stature but adult performers as well, which seems unreasonable... plus if I were to guess the performers themselves would generally not want to share a platform upon which they benefit with such things...