This is not necessarily true. Those who study law may not be able to pass the judicial examination. I have two views:

First, whether the professional level is high or not does not depend on whether you have a diploma in this major (or whether you are a major in this major). They are not good at learning, and some people who were not originally majored in this field have very high standards. For example, Chen Qizhen is actually a top student in the philosophy department, but she composes and sings her own music and plays some instruments by herself. I am also an art major, but Now I have made an album (not to mention how good I am), and I am also doing arrangement and teaching work.

The second is that even if you know it yourself, you may not be able to speak it well enough for students to understand it. For example, there is a swimming coach in the United States who can’t swim himself, but he actually trained an Olympic champion. And some people are very talented and knowledgeable, and the things they make up are very good, but if they are asked to explain how they make things up, it is really difficult for them. Art does not require many skills or inherent knowledge. It is a creative work, especially composition and arrangement. To a large extent, it has to do with personal feelings, which is the so-called inspiration and talent. A music master once said that composition cannot be taught (it can only be learned, and there is no general "teaching" in other disciplines).

Hope this helps!