Everyone has a six pack.
It just depends how much fat there is on top of it, most people you can't see their stomach muscles.

Although Beyonce most definately does work out, she probably couldn't reduce all the fat on her stomach without reducing other areas, (you can't spot reduce).

Seeing as she is young and fit, her skin wouldn't be saggy from lipo.

Anyway, who cares, I'd get lipo too.