If writers aren't learning new words, then readers can't learn from them. When do writers learn new words? From reading! Who writes what the writer reads? Writers.
King seems to be saying "the best way to discover new words is through the labor and chance of picking up the right books and finding some words you had not read before". So all words that can ever be useful have already been written or will be invented by fiction writers, and it is up to you to read a variety of styles and types of fiction rather than the compendium on your shelf. I find this notion silly.
The answer of course is a blend. If someone is leaning on a thesaurus to make bad writing good, there will be a problem, too.