We have all heard that other line that sounds similar to this one. Of course, it’s false. Plenty of people teach who can also do. I was literally paid for years to both perform engineering analysis work and to train others how to do it. It wasn’t a dichotomy of do it or teach it, as is often portrayed.
Teaching is a way to ensure that you have mastered a subject yourself. The amount of questions needed to be answered, alternate ways of describing complex concepts in a manner that allows different people to grasp them, ideating examples that describe the edge cases, and so on, all lead to mastery of understanding.
Teaching is the act of making a test for yourself in your own understanding. Someone training a new employee is checking that they themselves understanding the workings of the company. Someone teaching economics is constantly checking that they can relate the concepts to world events for their students and that the understanding and principles still hold.
Whatever you teach, you are also a student of. When you teach, your pace of learning may slow down as your approach the pinnacle of available knowledge about the subject, but your students will never let you stop learning entirely.
As my friend and coworker, Sam Hochberg says when being asked how the class he taught went, “Great! I learned a lot!”.
You can learn too. Just teach what you can. Students are everywhere.