The slippery slope claim made in the past was that they were only looking for files that match known examples of kiddie porn.
For instance, this tech from Microsoft:
>PhotoDNA creates a unique digital signature (known as a “hash”) of an image which is then compared against signatures (hashes) of other photos to find copies of the same image. When matched with a database containing hashes of previously identified illegal images, PhotoDNA is an incredible tool to help detect, disrupt and report the distribution of child exploitation material.
https://www.microsoft.com/en-us/photodna
Reporting parents to police because they took pictures of their child's first bath and those photos were automatically backed up to Google servers is much, much worse.