It's kind of ironic that this is a saying because generally those who have actually experienced and studied a topic tend to be the best teachers in my opinion. For example, if you want to learn about finances, then you want to learn from someone who has actually become wealthy and is debt free versus someone who has only been to school and studied about it.
Or, one of my best lecturers at University was an emerald mine owner and millionaire until he lost it all. He had some lessons to teach.
Agreed! But by far anyone who has been in the field whether they succeeded or not is a great teacher in my opinion.