Exactly, so I'll reiterate: if all you know is that setting A is opt-out and that user B has that setting turned on (i.e. in its default value), it's incorrect

Exactly, so I'll reiterate: if all you know is that setting A is opt-out and that user B has that setting turned on (i.e. in its default value), it's incorrect to say that the user has opted into the setting, because you don't even know if the user knows the setting exists, and one can't make a choice about something one doesn't know exists. The most you can say is that the user has not opted out of the setting.