The law doesn't require them to actively go around scanning for it.
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.