the bigger issue is this IS a backdoor, they promised (and still do right on apple.com in a pledge) to NEVER create backdoors because they admit it can be abused. if every single one of your photos is hashed and stored on icloud then at some point you have to trust that apple won't look for DIFFERENT kinds of photos, perhaps an "inappropriate" meme or political photo etc. to lump you into the "undesirables" or something. i know it seems far fetched but apple refused to create backdoors to help get data from known terrorists trying to attack us citizens on our own soil so they seemed pretty staunch but suddenly now they say it is ok... well shoot, it is worth looking into.
“Photos could theoretically be hashed to be compared against hashes of other photos” is not a technical backdoor.
It’s just a precedent, an idea.
Apple being the most privacy conscious (while not perfect at this) big tech is also a precedent.
Weight the precedents against each other to infer what will come next.