Some of the research, including this paper, is trying to get at the question of whether a species' sensitivity to the bouba-kiki effect might be at the root of language or not. Since it seems accepted that chickens do not have language in any meaningful sense of that term, finding that they still show this effect decouples it from "the origins of language".

You do need to research "obvious" things every once in a while. They have this annoying tendency of being proven wrong occasionally.

It's a hypothesis. How would you prove or disprove that it's because of that? (and I would say, a priori, it's not utterly obvious that the brain would relate spacial and temporal frequencies like this)