I respect these words because they came from a man deserving respect.
By illusion he means the feeling of authority on a given subject – yet you do not have command on the subject. You have the feeling of knowing and in return you don’t muster to have more knowledge. So by the time the know all feeling strikes your mind you don’t strive to gather more, so the enemy is probably little or not enough knowledge.
This is closely similar to this quote that I found said to have been made by Daniel J. Boorstin:
“The greatest obstacle to discovery is not ignorance – it is the illusion of knowledge.”