With a steady-state sinewave, a 180-degree phase shift is indistinguishable from a polarity inversion (or "complement"); therefore lots of people make the (false) assumption that "180-degree phase shift" and "polarity inversion" are synonymous.

A signal with a duty cycle not equal to 50% is an excellent illustration of the why a "180-degree phase shift" and a "polarity inversion" are not synonymous!