Children's book author here. 13 books. Mostly middle grade. I have never dumbed down my language.

Yet I have heard this insipid advice from many writers.

As you already pointed out with the infamous ten dollar words debackle, people misunderstand Hemingway's supposed preference for simpler language. That advice came out of his spitting match with Faulkner. The only reason why he attacked more complex language is that Faulkner accused him of lazy writing.