I do not know who those experts were but the experts that study the methodology of WSET tend to have a bit better luck. And jammy is not a tasting descriptor so I suspect they were self-proclaimed pros or were using a standard other than the wold wide benchmark of WSET. It is very hard to fathom that anyone who has proper training could not differentiate between grapes as that is one of the very basic skills and one which uses indicators that even amateurs find useful.