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.

I asked as an aside to the matter of control group - what is considered "given" about "the method of measurement" ?

According to this professional medical trial, the recorded ASD scores are "based on the Childhood Autism Rating Scale (CARS) rated by a professional evaluator"

You should have specific knowledge of that evaluation in order to criticize it as relying on "subjective questionnaires" and shouldn't introduce doubt by simply stating something as "given".