To be fair the Twitter thread says (emphasis mine) "These tools will allow Apple to scan your iPhone photos for photos that match a specific perceptual hash, and report them to Apple servers if too many appear."

I don't know what the cutoff is, but it doesn't sound like they believe that possession of a single photo in the database is inherently illegal. That doesn't mean this is overall a good idea. It simply weakens your specific argument about occasional false positives.