Firstly, I still don't see what the humor was supposed to be.I think you should reboot your ‘sense of humour’ algorithm, Gerry Seymour.
But since you’ve asked, martial arts is the one of the few physical disciplines where the teacher is expected to be ‘better’ than his students. It’s very strange because nobody expects a boxing or swimming coach to be as good as their top student but, for some reason, there is that expectation in our disciplines.
As a complete aside, most people get fat as they age because their calorie intake exceeds their metabolic needs. This happened to me so I reduced my caloric intake and KAPOW! Normal range BMI ensued. There are some medical conditions that can make the person put on weight, hypothyroidism and Cushing’s syndrom but the latter is quite rare. ‘Bad genetics’ making everyone fat is an excuse for the vast majority of people. Rarely, a pattern of inherited obesity within a family is caused by a specific variant of a single gene (monogenic obesity)
The difficulty some people have because of their genetics and/or endocrine issues is more common than just including like Cushing's and hypothyrodism. It's almost certain there are epigenetic issues involved, too, though I've forgotten what the research says about that. I do recall that both previous personal and parental dietary habits have been shown to have an epigenetic effect that causes the body to store fat more readily (and use those reserves less readily).
What worked for you will absolutely not work for everyone. I'm genetically lucky that I'm able to maintain my weight with little effort (I just have to avoid extreme overeating). Where my genetics make it easy, others do the opposite.
As for the other part, I wholly agree. It's odd that so many in the MA community tend to expect the instructor to be the best fighter in the room. I certainly had at least one student who could keep up with me in strikes-only sparring (he had prior experience), and if I'd managed to keep the program going, I'd have hoped that by now I'd have students who could best me in other areas. The best fighter is unlikely to be the best teacher - they are different skillsets. It's pretty rare to find someone who excells in both.