The "Don't take nutrition advice from a fat doctor" is to me, exactly, like my examples above. That guy is saying: don't take advice from people who, obviously, don't follow their OWN advice. If the teacher doesn't follow their own advice, why should anyone else? This seems very obvious, to me.
It seems like you have a narrative, and you are trying to bend the story to fit that narrative. I think if we're being honest, it's kind of obvious that you wouldn't hire a fat trainer at the gym, given the choice. You wouldn't hire an accountant who is broke. You wouldn't hire a Shopify Guru who doesn't have a successful Shopify store. That's just common sense.
When asking the question: "Why should I hire you?" - the answer should be "Because I've used these principles myself, and \here* is the proof that it works, in my own life." But instead, that answer is "Well, I know these principles *should* work, but once you pay me, we'll find out together!*".
It sounds like instead, you are virtue-signalling and saying "Nope! I will absolutely give my hard-earned money to that person, even though they have not shown they are capable, because it would be an tu quoque fallacy!" I mean, you can spend your money how you want, but most people won't waste money, just to make a principled stand.
Am I missing something, here?