As per the provided link, there's a number of factors going on.
First of all, a kiai is as much about a forceful exhalation and a focus of purpose as it is actually yelling. Many of the fighters do just that grunting or exhaling on a heavy strike.
One of the other purposes of a kiai is to startle the opponent. In the Octagon, most fighters are prepared for ...
If you read the more recent studies (which the NFL hid for a long time) they find that head trauma is additive and even mild hits can lead to damage. If you go to pubmed.gov and look up Mild traumatic head injuries you will get many peer-reviewed articles on that. hope this helps