Autism, like many other mental conditions, is not very well understood. For example, diagnostics is problematic, because the classification changed recently, and the diagnostics protocols involve observers of childhood development, which not every adult can provide. So, additional signs like the one described in the article could help people get diagnosed in the future. It could also help people hunt autistic people. But that's more of a problem with those people, and not with knowledge itself. Most pieces of knowledge can be used to hurt others.

You're right that it can be difficult diagnosing children but that is why, at least here in Europe, kids are observed by medical professionals along with their parents over several visits to child and adolescent psychiatric units. Sometimes they are even admitted to get even closer to a correct diagnosis. Also, almost all parents will seek help very early if they have any suspicion of disease in their child which makes this tech useless and probably stigmatizing to kids and families.