Calling someone a life sized Barbie is a sexist comment, but writing the comment makes the commenter sexist in the same way that playing the T-1000 makes Arnold Schwarzenegger an android from the future.
Still, you're not getting it. The commenter is speaking from the perspective of the type of person who would do this. They are doing so to mock that type of person. If you give the commenter the benefit of the doubt, assuming that they are not on the side of the rightwing psychopaths they are explicitly criticizing, then it makes sense.