Only like ~15 diet drink drinking mothers of females with autism were included in the study. They said they didn't show a link, not that they demonstrated a lack of a link. And I believe none of the links shown for males were claimed to be causal but were hypothesized to be.
To control for demographics they divided that ~15 into 6 different demographics (or 5 if male/female was already included, about 3 people per demographic if so).