The study evaluated the severity of autism symptoms (using subjective questionnaires) before and after the treatment, but didn't compare those results to a control group of people who weren't treated the same way.

It's like finding a couple dozen people with the flu, feeding them chicken soup, and then reporting that a month later they feel healthier, so the soup must have helped. Maybe it did, maybe it didn't, but you didn't measure that so you don't know.