As for the president forgetting the constitutional law he once taught, even fully remembered it's not much better than hitler teaching an ethnic diversity course.
As for the President having taught constitutional law, it is accepted that he did teach a course, but I don't believe I have ever seen, heard or read anything about
what he actually taught about the Constitution in those courses. Probably something along the "living document subject to interpretation according to the current mores" line, I expect. Not likely he was teaching as an originalist or strict constructionist.