At first reading both statements seem mutually exclusive. However, there is a way for both views to be consistent if one considers that maybe Sagawa stated that he purposely held back knowledge because he didn't want to admit that he lacked the ability to clearly describe what he was doing.
Except that as was mentioned, when Sagawa chose to share some of his training methods with his students, they immediately started to make progress and improve in their abilities. His student Kimura, who faithfully and diligently trained in what Sagawa gave him, progressed further and at a quicker pace than his fellows. Evidently, Sagawa was quite capable of conveying his training methods effectively to others when he CHOSE to.