They were thinking that they hired a firm to accurately predict how their product would be received by the flavor receptors and this company came up with a method that works. Pepsi is by no means the only company that does this, flavor predicting is big difficult business.
There is no substitute for the old fashioned taste test, if finding good flavors is the key.
What is weird is that these were not taste bud cells being used. These were kidney cells. It appeared to me that they were being used to verify the presence of trace chemicals that add nuance to flavor, so as to maintain the flavor quality of existing formulas.
Also, most of the companies that were using this service bowed out when people asked them to but Pepsi refused. I guess they found out that their customers opinions matter.