Gould actually finds two vulnerabilities in this paragraph.
The first vulnerability is in Broca's failure to take into account the different heights of the men and women whose brains he weighed. Gould says that Broca knew that the size difference in brains might be partly caused by the difference in body size and yet he did not try to control for that.
The second vulnerability comes from the reason why Broca did not take height into account. He did not do this because he already "knew" that women were less intelligent than men. He was supposed to be studying brain size to see if women were less intelligent, but he then says that he already knows they are. He uses the "fact" that he knows women are less intelligent to explain why he doesn't need to take their size into account.