This is a great question. As you note, some researchers do believe that there is a strong biological component to temperament. However, no one believes that this is the only component. All believe that that experience and nurture also plays an important role. This last point should not be forgotten. In addition, some researchers argue that the most determinative component is actually life experiences. So, the jury is out on this one. With that said, those who argue for a strong biological component to temperament come to this conclusion in two ways.
First, they often state that babies right from birth have very different personalities. Some cry easily; others rarely cry and the list goes on. They fact that these babies have very little life experience and yet act in a certain manner suggestion that they were born with a certain disposition. This, they argue is temperament, which is biological.
Second and more substantively, researchers use twin and adoption studies to argue that individual differences in children's temperament are biologically influenced. In other words, those genetically related have similar temperaments, while those who are not differ more readily.