Think about it: almost every nontrivial conversation you’ve had or comment/blog/article/book you’ve read constituted an entirely new (to you) utterance which you understood and which enabled you to acquire new ideas and information you had previously lacked. No non-human animals have demonstrated this ability. At best they are able to perform single-symbol utterances to communicate previously-understood concepts (hungry, sad, scared, tired) but are unable to combine them to produce a novel utterance, the way a child could tell you about her day:
“Today the teacher asked me to multiply 3 times 7 and I got the answer right away! Then Bobby farted and the whole class was laughing. At lunch I bit my apple and my tooth felt funny. I think it’s starting to wiggle! Sally asked me if I could go to her house for a sleepover but I said I had to ask mom and dad first.”
We maybe disagree, in the sense that it seems to be mixing indefinitely bounded expressiveness with actual unlimited expression production that could potentially be in a bijective relationship with the an infinite set of expression.
We human are mortals and even at the whole humankind scale, we will produce a finite set of utterances.
The main thing bringing so much flexibility to languages, is our ability to reuse, fit and evolve them as we go through indefinitely many inedit experiences of the world. So something like context change tolerance. But if we want to be fair with crediting admirable unknowingly extensive creativeness, we should first consider the universe as a whole, with its permanent flow of novel context, which also include all interpretations of itself through mere mortals as ourself.