Gould points out many deficiencies in Broca's work on women's brains. Most importantly, he points out that Broca did nothing to try to explore other reasons why the brains of the men he studied were bigger than those of the women. Gould points out that Broca, in effect, said "men's brains are bigger because men are smarter." But Broca failed to note that the men whose brains he studied had bigger bodies and therefore ought to have bigger brains. He failed to note that the women studied tended to be older and therefore would have smaller brains because brains shrink as we age. He failed to note that many of the women died of diseases that would have caused their brains to shrink as well.
In short, Gould is pointing out that Broca did not explore alternate hypotheses (other than his own hypothesis that men are more intelligent than women) that could have accounted for the fact that the men's brains were bigger than the women's.