Interesting answer, scanning through the Wikipedia article on kiki/bouba it makes sense that we don’t really have solid evidence that it isn’t a learned trait. It may be hard to get a population of people who developed language independently of all other humans ever and see if they maintain the strong correlation with naming kiki and bouba.
So I guess that brings up another question I have kinda wondered about. What is the most “isolated” spoken language on the planet? By that, I mean the language that evolved most independently of other spoken languages. Is there anything interesting that can be learned by comparing such a language to the European languages that are dominant among the global population?