Why is someone's vocabulary a signal of their intelligence?
Vocabulary breadth as evidenced by a person's speech is an excellent metric for intelligence. Speech represents what a person has overlearned. A person who uses precise vocabulary correctly when speaking and whose speech is similar to standard academic English is generally able to produce language that is more sophisticated still when writing. You speak the sort of language you are deeply fluent in and no one can be an accomplished, articulate speaker without being intelligent.The distinction between vocabulary words and concepts is artificial. You don't speak using words you know you don't understand, and you can't understand many words without understanding the related concepts. Take the common science term evaporate. Who would use that sort of word metaphorically when they speak without also understanding conceptually what it refers to? No, you reach for that word precisely because you understand it and believe it's the best tool for the job.The smarter the speaker, the better the toolkit.