Perhaps she was afraid that if she simply used the slang the way you were using it, she would be accused of cultural appropriation. I think you're right, she should've just recognized that you were all speaking the same language, and have the same background. But the whole idea behind the condemnation of so-called cultural appropriation is that if you have the wrong DNA, then you are not allowed to use certain symbolic constructs--words, clothing, music, hair styles etc. If she's not allowed to use them, the only way she can participate in the group is by joking about her being forbidden to use them.