She's not complaining because they referred to her as a singer; it's her job. She's mad about them referring to her as "Harry's ex girlfriend" and "Calvin's rumored girlfriend" in the article (i.e. repeatedly referring to her as if she's different people's properties), which is why she brought up the feminist part.
Unfortunately that's how she presents and defines herself to the public: according to her relationships.
Also, it's OK Magazine. Why is she expecting great journalism from them
a Kesha tea