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.
I agree with King on that. A thesaurus is great for finding words you know but don’t use often. Learning new words from it… you’re usually missing important nuance.
King is right about that. If you want to become a better writer, you need to read more good books with high quality prose and stop reading garbage. The thesaurus, while sometimes useful, can become a crutch. Stylistically the great writers will rub off on you if you read them carefully.
John McPhee takes a different approach. Whilst he cautions against over-reliance on the thesaurus, he's a full-throated advocate of a good dictionary:
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.
The first sentence is about the writer; the second about the reader.
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.
There's a huge potential collection of words you can use. But there's also the words in the current zeitgeist that will be meaningful to your readers.
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.
I am sure this is out of context, but I regularly use a thesaurus to find names for patterns in my code/design/architecture. Applies to class names, variables, even functions.
I don't think King was saying "never use a thesaurus", he was just pointing out that relying on it too much is not a good way to develop as a writer. The way you do that is by reading great books.
I often use thesaurus to translate complex words into simpler ones, it really does improve the overall writing especially if you are biased towards redundant complexity ...