Deaf children can be taught to speak by very explicitly demonstrating tongue/throat position. It's a pretty arduous process and has fallen out of favor, so most deaf people who don't get a cochlear implant will use sign language only.

However, with early implantation language acquisition is relatively easy (thought it varies per child).

The sensory medium is separate from one's capacity to learn and use language. Sign languages have grammar, vocabulary, "accents" etc just like spoken languages.

I think they mean how can a person born deaf learn to make speech if the above quote says this individual will not be able to speak a language. I think the answer to that is it's more "they won't be able to make speech like a person born with hearing would do by listening and naturally learning" rather than "they won't be able to try to make sounds with their voice they are not able to process auditorily".

Moreso comprehend it. It seems impossible to suggest that a person could learn to vocalize language but not to understand those vocalizations. It may be impossible in spite of what it seems.