This is why the body work has to come at the beginning of a person's training, to create a foundation that will become the person's "true nature." Whatever he has wired in, is what he will fight with under duress. It is very difficult to un-do old training and replace it with another completely different method. And, it takes a very special ability to deconstruct an old foundation and build a new one; you have to very open and willing to break yourself down to build yourself back up differently.
I suspect that those people you mention, began their training with a more conventional "external" method, and came to internal training later in their careers, adding it to their arsenal rather than reconstructing themselves with it.
To reinforce this point and to borrow a phrase from Chinese martial arts, using taij as an example - "Taiji is easy to learn but difficult to correct"