I can see why you would think that way. But reading the article i don't think that's what she's saying.
I think she's talking about people, beyond the scope of abdl by the way, that have grown up and mature normally just like anybody else but have retained a part of their younger selves.
I can certainly identify with this, i've always felt comfortable with my age but at the same time i feel like i'm still imature and silly as well as being mature, i've also had the desire to regress and feel younger since i was 3.
Also, now that i think of it i think prefer the term advanced baby rather than adult baby to be honest