Is this really a goof?

"Huey actually blows his cover for pretending to be deaf in front of Mr. Smith, and gets away with it. When Smith tells Huey that he remembers a shoe shine to cost two bits, Huey responds with a head nod and a slight grunt."

But aren't some deaf people good at lip reading or maybe his head nod and a slight grunt was like a polite sign of appreciation after providing a service rather than an indication of having heard what was being said?