Honestly had never even heard of it! But adult language acquisition isn't really a domain of study I've ever been interested in. I can only speak to what I have found most helpful in my own adult language acquisition journeys. The journaling method was taught to me by a polyglot friend of mine and it sort of solved the "what actually is my everyday vocabulary anyway" side of language learning for me.

tl;dr "The Foreign Service Institute (FSI) is the primary training institution to prepare American diplomats to advance U.S. foreign affairs interests, teaching, among other things, the languages of the countries where Foreign Service Officers will serve. "

Apologies, I should have linked beforehand.