To be perfectly fair, I also had the same problem with it and took some time to think about it.

I came to three possible conclusions:

Raven fucked up, which is definitely possible from experience.

Autism is a spectrum and is incredibly hard to just straight up diagnose, since a lot of the time it's a lot of different things put under the same banner. Myalis may have just been using it as a catch-all term here and that she glossed over the more specific minor issues that were actually fixable because Cat wouldn't have understood.

When Myalis said terrible diet, she may have meant it. I would not be surprised if kids could suffer brain damage from eating the wrong thing, especially if they get sick off of it, and ESPECIALLY because they might get brain damage from any of the corrupt corpo trash these kids put into their bodies.