I definitely agree that women should stop holding in their stomachs, and shouldn't be pressured into thinking they (or anyone) must have a flat midsection. But the abstract you cite in this paragraph actually says the opposite of what you state; (cisgender) men store more fat in the abdominal area. As a transmasculine person, I can attest to this; testosterone therapy gradually shifts fat accumulation from the hips and buttocks to the waist area.