This is clarified in Chronicle. Cenarius did live among yaungol, the ancestors to the tauren, when they settled near the Well of Eternity between the 12,000 and 11,900 BDP, but Malfurion was the first to be trained in the ways of the druid, around year 10,000 BDP. This is why Malfurion is referred to as the first mortal druid.