maybe by "deaf" she means hearing people are able to understand very little about what is being communicated from the neck down, much like how deaf people may not be able to understand much about what was communicated from the neck up.
tl:dr; deaf needs to see neck down to communicate, hearing needs to hear neck up to communicate.
but actually, what about sign that are above the neck, like mother, father, president, understand, know, wake-up, deaf, home, sleepy, black, silly, flower.. etc?