[tsimionescu]

The idea that language is about thought is only referring to the kind of higher level thinking specific to humans. Any animal can catch a tennis ball, but only humans can construct and then execute complex plans of actions.

This line of thought stems mostly from two observations: one, that the vast majority of language you use is your internal monologue; and two, that this internal monologue would have been extremely helpful for the hominids that would have first developed it even while the rest of the population had not developed language (in contrast, if language is about communication, then it's only useful to a hominid if the whole population speaks language, but then it's hard for it to spread initially).