According to the numbers from the press release, periconceptual folic acid use by the mothers would have a relative risk of about 0.76 (a relative risk below 1.0 means that whatever it is you're looking at appears to have some protective benefit), and would probably be statistically significant. But very few children in either group were diagnosed with autism. Only 0.10% of children vs. about 0.21%, so the actual clinical benefit is pretty small. Also, this benefit was only seen when women took the folic acid around they time they conceived; there wasn't any benefit if they didn't start taking the folic acid in the middle of the pregnancy.
As with any large population study, there may be other reasons for the results and not specifically the one being studied. In this case, the mothers had fewer autistic children may have had other things in common that affect autism risk. According to the study, the women who used folic acid within the exposure interval were also more likely to have college- or university-level education, to have planned the pregnancy, to be nonsmokers, to have a pre-pregnancy body mass index below 25, and to be first-time mothers. Another important note: a study like this only shows a correlation between folic acid intake and risk of autism, it doesn't prove that folate deficiency causes autism, or that taking folic acid will provide any benefit to children who have autism.