No. You cannot rule out the possibility that women discriminate women, too, or that environmental influences from outside the work floor cause the effect.
If women discriminate then we should know this as it would be very helpful to know why.
If the cause is environmental influences outside the workplace then it is a bit hard for anyone to solve from within the workplace. It also makes any hypothesis of sexism untestable since there is no possible evidence that could be gathered that could ever disprove sexism as the cause.
If the cause is environmental influences outside the workplace then it is a bit hard for anyone to solve from within the workplace. It also makes any hypothesis of sexism untestable since there is no possible evidence that could be gathered that could ever disprove sexism as the cause.