Anyone involved in neurolinguistics is going to differentiate the skills involved in learning to speak words and learning to write letters from language development.
Of course writing letters is connected to learning written language, but knowing how to write is not the same as knowing English (in fact, some people type, some dictate, some write other languages). Writing is a skill that can be employed in expressing the language, but it is not involved in receptive activity, reading written language.
Similarly, oral skills are connected to learning spoken language, but knowing how to speak is not the same as knowing English. Oral skills can be employed in expressing the language, but are not involved in receptive activity, hearing spoken language.