I can see how it might happen. I am sure most Ts do learn about attachment theory, developmental trauma etc during study. But if they haven't experienced those things personally (and many wouldn't have) I can see how it would be quite one thing to know about them in theory but be completely thrown by their intensity and management in practise. Experience in the field can only come from experience in the field... which means that most Ts would spend a good deal of time doing things wrong as they 'feel their way through' before they manage to 'get it right' more consistently. I think there would be a certain amount of learning 'what not to do' involved. I don't know if there is any other way of 'learning to T'.
I also think there would be Ts who are better at learning on the way than others. Some people can learn from and admit their mistakes, and others can't. That would be a crucial characteristic for a 'good T' I reckon.