Right? I have found that teaching is a skill like anything else and is usually orthogonal to whatever skill is under discussion. It'd be like saying "Those who can, do. Those who can't, bake."
I think a more useful saying would be "Don't assume that just because someone can DO it, that they can TEACH it." i.e. Engineering professors :-(
The best teachers that I've had, were trained as teachers. They knew the material, but weren't application experts.
The worst teachers that I've had, were content experts. I remember this Calc II teacher, who was a genius, but had no patience at all, for people that weren't on his level (we call those people "students"). I generally ask a lot of questions; sometimes, ones that seem obvious. I end up learning the material very well, but the process can be a bit messy (and embarrassing). This guy would abuse people that asked questions.
I got an incomplete, that semester. I was on the Dean's List, all the others.