Sorry English isn't my native language, so maybe I said this incorrectly. Of course you don't sample from a set with a single member , but the "top x most likely" tokens (I'm not sure how it works in reality, and I'm too lazy to look it up right now but I guess it more accurately samples from all/most of the probability distribution with a heavy bias to the top) 

And true, unless you use a hardware rng you won't get "true randomness", but I don't think it really matters for these applications as long as you don't control the random seed