In this use of the term "scans of healthy brains" means that the people who were scanned had no known diseases or disorders. That means no cancers, no exhibition of seizure, no migraine, nothing that would interfere with those scans as a control to compare the scans of people known to have ADHD and/or autism.

In other words, it isn't the same language as we might use colloquially. It's specific to science writing.

Likely, the original data said something like normative, or unremarkable, or some other more tightly defined term that just isn't as accessible to the broader populace.

Is it an ableist term? Ehhh, it could be argued either way, but I tend to look at jargon as being almost completely different words from standard versions of a language. It's more like African-American vernacular English, or Appalachian English than formal English; some things don't translate well. Hell, look at medical terminology as a whole; it's a hodgepodge of latin, Greek, and little fragments of other languages mixed into the dominant language of wherever it's being used.

So I wouldn't say the article's usage of it is ableist either in intent or in function. It's like saying something in Spanish that's similar to a slur in the way it sounds, but utterly lacks the context and meaning.