None of my photos have been scanned, nor ever will be unless I choose for them to be. I don’t have to do anything to achieve this. They won’t scan anything unless I decide to go ahead.

That is the very meaning of opt-in.

Opt-out typically means that someone will go ahead with something unless you decline. This is not that.

I do agree that if I don’t want on device scanning in future, I will need to choose another could photo service, but in the meantime, nothing will be scanned without me taking positive action to initiate it.