In your first sentence, "She is not so excited to see him, but you are", so has a similar meaning to really. In fact, it would be more natural to use some word such as 'really' or 'particularly' rather than 'so' here, in my opinion.
We cannot really say that she is not as excited as anybody else, because she is not excited.
It is, I suppose possible to argue that she is a little excited (though it sound slightly self-contradictory). If you believe that, then you can say, "She is not as excited to see him as you are".
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