I don't know if it's a hard-and-fast rule, but if someone majors in two subjects, they would have to have graduated with a dual major in order to be able to say they have a degree in each.
Like AOC, I majored in two subjects, but like her, only in a major/minor sense - not a dual major. I got a BA in music with a minor in philosophy, but I never say I have a "degree in philosophy". My friend, on the other hand, got a dual major, in Computer Science and Physics. I believe he can say he has a degree in either subject.
If AOC majored in international relations with a minor in economics, I think it's incorrect to say she got a degree in econ.
I'm not sure about that. In my experience, a minor concentration requires as few as 15 credits out of a 120-130 credit degree. That seems like a thin educational background for a "degree," doesn't it?