This argument is a Russell's Teapot. Theoretically that's possible, but it's unparsimonious to assume that's the case when environmental factors already explain such a huge portion of the behavioural variance.
Do you have a reason for thinking there's some undiscovered metric or neuroimaging technique that will show massive differences where none of the currently existing ones do? And if that's the case, why is so little of the behavioural variance between sexes still unexplained? Wouldn't a large difference in brain structure discovered with a new method be extremely surprising, given how small the actual behavioural difference between sexes is once environment is accounted for?