If there were no false positives there would be no legitimate reason for Apple to review-- they would just be needlessly exposing their employees to child abuse material.

But the fact that there is no legitimate reason according to the system's design doesn't prevent there from being an illegitimate reason: Apple's "review" undermines your legal due process protection against warrantless search.

See US v. Ackerman (2016): The appeals court ruled that when AOL forwarded an email with an attachment whos hash matched the NCMEC database to law enforcement without anyone looking at it, and law enforcement looked at the email without obtaining a warrant was an unlawful search and had AOL looked at it first (which they can do by virtue of your agreement with them) and gone "yep, thats child porn" and reported it, it wouldn't have been an unlawful search.