Non-speaking doesn't mean non-thinking
Sam is an unreliable speaker. That is the phrase we use, because it says what is true: speech comes and goes for him, and it never carried his full thoughts.
Non-speaking describes the outside. It says what a person does not do. It says nothing about what they understand, notice, or feel.
Sam was listening and understanding all along. He heard the conversations about him. He formed opinions about them. He had humor and insight and a clear sense of who people are. Speech was the one channel that did not work well, so the rest stayed hidden.
We learned to listen in a new way. Spelling gave Sam a channel that fit how his mind works, and his words did the rest.
Speech is one channel. Sam's story shows how much more there is.
Questions
Is Sam nonverbal?
We say unreliable speaker. Sam uses some speech, and spelling carries his full thoughts.
Why does the language matter?
Words shape what we expect from people. "Non-speaking" can sound like "non-thinking" to a stranger. Sam's story is proof that the two are different.