These experiments were first conducted by the German gestalt psychologist Wolfgang Kohler and were repeated with the names "Kiki" and "Bouba" given to these shapes by VS Ramachandran and Edward Hubbard (the paper you linked to). In their experiments, they found a very strong inclination in their subjects to name the jagged shape Kiki and the rounded one Bouba. This happened with about 95-98 percent of the subjects. The experiments were repeated in Tamil speakers and then in babies of about 3 years of age. (who could not write) The results were similar.
Humans are more synesthetic than we imagine.