E is incorrect here because the professor is trying to prove the argument wrong and the argument is saying the following
IP -> FT
The professor's reasoning is
IP -> ID
So stating E, which is diagrammed as FT -> ID, simply makes it such intellectual discipline is another necessary condition for freedom of thought. The entire argument would become.
IP -> FT -> ID
This would not prove the argument wrong. It would just add another necessary condition.
I hope this helps. Please let me know if you have any other questions or would like to expand further!