They’re scanning for it as it enters their system. It’s only scanned if it’s set to go to iCloud.
Apple is being its usual cryptic self about this, which is once again breeding uncertainty, but I still have hope in the end this will work out.
However since this is part of the opt-in iCloud photos sharing mechanism, it doesn’t appear they have started to exfiltrate data without consent.
I don't in know why you're assuming they have a backdoor built in already. The whole point is that they don't, but they're going to add one.
So they have always been one software update from non-opt-in data exfiltration and remain so.