Because it's a family trait?
Khelif was identified as female upon birth, it's on the birth certificate. Raised as a girl, obviously, and as compared to other members of family who were raised as males as well as the father they most closely resembled, well....
That's actually a shadow under the chin. Not that women don't have Adam's apples, because without one, they wouldn't be able to speak. It's just that the larynx is more pronounced in males due to increased testosterone expanding its size during puberty. If you notice, it isn't exactly always that readably noticeable in most men either, unless they're on the thinner side.
In Algeria, it is illegal to be trans, it's even illegal just to be gay.
So if Khelif is actually biologically a male, then it's something everyone was unaware of since Khelif was born.