A craft takes specific skills, rules, and standards to follow. Art is very subjective in its presentation, without rules. Maybe writing can be both. Grammar, spelling, syntax, etc. is the craft while putting words together creatively with feeling, tone, and voice constitutes the art of writing. I dunno. Just thought your question was interesting.
I think it means that like so much of everything else, good writing has an element of technique for most people. Good principles of writing are learned. They are a skill that needs to be honed. I do feel there are some natural writers, writers that have art in their blood, but for most, good writing must be business, or a means of making a living. This kind of writing must be constantly studied and updated to be relevant and lucrative. Especially pertinent for writing today, as internet writing is certainly more of a skill than an art. SEO is learned and needed and internet writing is direct, clear and as colloquial as possible.