Actually, it's a misleading title. There's association, but there's no proven causation. Here's the quote:

>This study is consistent with other studies on the same research question. Each study is observational because randomized controlled trials are not ethically possible during pregnancy. Hence, each study is describing an association. The accumulation of such findings will lead to causation

It's observational study, i.e. a study which can only show association. You can easily think of many reasons why you might get such a result. For example, depression and autism might have overlapping genes, and having such gene, results in depression in mother and higher risk of autism in a child.