That's one theory. Another one I can think of is that sharp edges are scary, and most distress calls are high pitched.

Also, the thing about high frequencies and sharp edges lead to a contradiction: babies are more round than adults and produce higher pitched sounds, this is almost universal across all species.

There are other tentative explanations, such as how the vocal tract acts when producing these sounds, with "bouba" sounds being the result of smoother movement more reminiscent of a round shape.

"kiki" is not just higher pitched, it is also "shaped" differently if you look at the sound envelope, with, as expected, sharper transitions.

So to me, the mystery is still there. Is is the kind of thing that sounds obvious, in the same way that kiki sounds obviously sharper than bouba, but is not.