It's a little strange for us to insist that a poem about hips would be about strength, right? Bear with us, though – these hips aren't just strong gluts and quads. Oh, no. They're a symbol of the speaker's inner strength. She's a free woman, just like her hips. And just like her hips, she refuses to be put into a stereotypical box – she defies convention. Much of her resistance to cultural stereotypes is articulated as personal strength – which is why she (and her hips) thrive in a world that might otherwise seem hostile to women.
Recognizing that her hips aren't bound by social conventions allows the speaker to access her own personal sense of strength.
This poem allows readers to see physical strength as a form of female beauty.