I actually agree with this a lot. I agree that everyone has something they specialize in. My disagreement with this comes with the idea that you cannot teach something you don't specialize in.One Shuai-Chiao teacher said, "If you have learned throw A and throw B from me, you can leave, and find yourself another teacher. I may know a lot of MA material, but I'm only good at throw A and throw B. Any other MA teacher can also teach you the other MA material as good as I can teach you". I like that teacher's "honest attitude".
When my teacher was young, he wanted to learn an old master's "door guarding skill". That old master didn't want to teach him. My teacher kicked on that old master's front door, cursed all his family members. The old master came out, used his "door guarding skill" to beat my teacher up. My teacher ran away and said, "Thanks for the lesson." Did my teacher want to learn the entire MA system from that old master? My teacher just wanted to learn the old master's famous "door guarding skill".
Most martial artists I know have some sort of specialty, or something that they excel in. Personally, I'm good at closing the distance, and retreating again. By that logic, this is what I should teach. And when I was teaching, it was because of this. I taught judokas how to close distance, to get into grappling range, so they could be more effective overall. My only issue is that teachers are not always the same as people who mastered the material. Sometimes you can teach a technique even if you have not had the repetition and practice to have mastered it yourself.
Personal example: I know how to throw a front kick and side kick to maintain distance until I choose to do my focus (engage, elbow/knee, disengage). However, I don't practice those as much anymore, so they are far from perfect. I could still teach exactly how to do them, and if I focused on teaching that (ex: If I was teaching self-defense, I would likely teach how to throw a front or side kick and sprint away from that) my students would likely surpass me in my kicking abilities.
The point of this is: you can practice what you like, and may be excellent at what you like. That doesn't impact your teaching abilities. If I practice a throwing art, like your example, I may excel at foot throwing techniques (Osoto Gari, Kosoto Gake, etc), but I should still be capable of teaching hip throwing techniques (O Goshi, Uki Goshi, etc.). Even if I have not practiced it enough to feel satisfied myself,that does not mean I can't teach it to someone else, to the point where they surpass me.
As stated, my focus is: Starting from outer distance, breaking distance with punches and elbows, utilizing foot sweeps, and breaking distance when failed. That is how I handle most sparring matches if my goal is winning. That does not mean that I cannot teach kicks, or punching while not trying to close distance. I know the technique behind those, and can teach it, it is just not what I personally focus on.