Their reported collision rate was against their CSAM hash database[1].

> In Apple’s tests against 100 million non-CSAM images, it encountered 3 false positives when compared against NCMEC’s database. In a separate test of 500,000 adult pornography images matched against NCMEC’s database, it found no false positives.

[1] https://tidbits.com/2021/08/13/new-csam-detection-details-em...

I don't follow the point you are making here. The goal of the algorithm is to match images so I would expect similar collision rates regardless of what image was matched against what set of hashes. The exact rate of collision will obviously vary slightly.

I think the point is that if Apple did optimize the algorithm using ImageNet then they and we are seeing the same best case scenario.

The only number I've heard from Apple is, "the likelihood that the system would incorrectly identify any given account is less than one in one trillion per year."[1] Which I read as enough false hits to flag an account that year (some interview said that threshold was around 30). That depends on the average number of new photos uploaded to iCloud, the size of the NCMEC database, the threshold for flagging the account, and the error rate of the match. Without knowing most of those numbers it's hard to gauge how close it is.

https://www.apple.com/child-safety/pdf/Expanded_Protections_...