It does not have to be misdiagnosis. If kid has both autism and gut issues, the gut issues could make the autistic symptoms worse, by causing distress to the kid, which could make the interactions with caregivers harder for both in a quite formative period. Treating the gut issues could help this way without gut being directly related to autism and without it being a misdiagnosis. It is telling that they report quite high (0.7) correlations between improvement in gut and autistic symptoms.

However they say they also have an adult trial running that seems to show similar effects, so there might be something more into it.