According to King, many writing teachers will not like the idea that competent writers can be made into good writers.
This is his exact quote:
"...while it is impossible to make a competent writer out of a bad writer, and while it is equally impossible to make a great writer out of a good one, it is possible, with lots of hard work, dedication, and timely help, to make a good writer out of a merely competent one."
So writers that have some merit will become good writers after a time of practice and hard work.