The phrase is a gross simplification of the truth. It probably refers to teachers of children only. I have been taught by teachers as a adult heaps of times. There are two types of teacher
1. Those that are good at teaching what they know. They teach as their job. However if you ask them a tricky question then they do not know the answer. This sort of person the phrase is aimed at.
2. These people are not really teachers. They only teach part time. For the rest of the time they actually do the work. They know everything or nearly everything about the subject. However they may not know how to teach.
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