Knowledge of punctuation is a writing skill, it has nothing to do with our unconscious knowledge of how the language works, I think.

I agree. Punctuation is a tool of Grammar. Until relatively recently my fathers native tongue was only an oral language and in his later writings he would never use a macron because he and others who knew the language didn't need to know how to elongate an 'a'. They just knew. But today, for the purpose of new language learners, punctuation (in writing) is essential in creating good grammar and context for the unintiated.