That assumption has been shown wanting by several remarkable non verbal people:
Joey Deacon, assumed to be intellectually disabled for 50years, until Ernie turned up & could understand him, they wrote a book, 'Tongue Tied' together with Michael, who could understand Ernie...
Jean-Dominique Bauby and his memoir, "The Diving Bell and the Butterfly" (Le Scaphandre et le Papillon), labouriusly written after he was 'locked-in', paralysed by a stroke, with assistance, one character at a time.
Helen Keller, prolific writer & disabilities campaigner, who was rendered deaf & blind by illness at ~20 months old.
Henrietta Freeman, non verbal disabilities campaigner, on twix. Uses assistive tech, writes in detail about consent issues & intimate care. Comes in for a lot of stick from Be Kind land.