Fwiw Stephen King in "On Writing" also says to not use a thesaurus and just use the words you already naturally know how to use. He follows on saying your vocabulary will naturally expand as you read more.
In the search for words, thesauruses are useful things, but they don’t talk about the words they list. They are also dangerous. They can lead you to choose a polysyllabic and fuzzy word when a simple and clear one is better. The value of a thesaurus is not to make a writer seem to have a vast vocabulary of recondite words. The value of a thesaurus is in the assistance it can give you in finding the best possible word for the mission that the word is supposed to fulfill. Writing teachers and journalism courses have been known to compare them to crutches and to imply that no writer of any character or competence would use them. At best, thesauruses are mere rest stops in the search for the mot juste. Your destination is the dictionary. Suppose you sense an opportunity beyond the word “intention.” You read the dictionary’s thesaurian list of synonyms: “intention, intent, purpose, design, aim, end, object, objective, goal.” But the dictionary doesn’t let it go at that. It goes on to tell you the differences all the way down the line—how each listed word differs from all the others.
<https://www.newyorker.com/magazine/2013/04/29/draft-no-4>
The point remains: to get the words right.
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.
The only way you can learn these words and give them a proper weighting of "usefulness" and understand their fundamental nature is to read. Yes, you can get definitions and similar words from tomes, and even examples of usage, but you don't really understand the connotations or colloquial uses until you have seen other people use them in full context.
Of course, there are times when one might want to dredge up an infrequently used word, coin a new one, or transport something from current spoken vernacular onto the page. This should be a tiny fraction of where our chosen words come from, though. Reading is the tool to build usable vocabulary, even if it isn't fetch.