Frankly the response only addresses a small part of the criticism (the definition of slope) and then launches an attack on the critic's motivation. The main point in the original article [1] was that Khan's preparation for his lectures is deficient ("I don’t know what I’m going to say half the time") and often resumes to "two minutes of research on Google".
[1] http://www.washingtonpost.com/blogs/answer-sheet/post/khan-a...