First off, not everyone "visualizes" thought. Second, what do you think "using a grammar model to turn those into speech" actually consists of? Grammar is the set of rules by which sequences of words are mapped to meaning and vice-versa. But this is implemented mechanistically in terms of higher activation for some words and lower activation for other words. One such mechanism is scoring each word explicitly. Brains may avoid explicitly scoring irrelevant words, but that's just an implementation detail. All such mechanisms are computationally equivalent.