It is a perfectly accurate little saying. If being accurate is a put down, so be it. And it is only a put down in the sense that they can not pretend to be great writers, artists or business people. They can still be great teachers, however.
It's simply a sneering put down by people who are actually showing how ignorant they are and not as they think, how clever they are.
You cannot teach what you do not know. And why is being a teacher seen as inferior to doing other jobs anyway, particularly as most people get to learn what they know from…teachers.
I saw a very good comic strip the other day about this ignorant attitude towards teaching. But cannot remember who did it.