It's like arguing that pneumonia can be cured through "healthy living" even after you've established the mechanism and effectiveness of penicillin.
I've even heard the "thinness is a sign of godliness" folks get really angry at the suggestion of using GLP-1 agonists, because it's "lazy" and "cheating."
We have existence proofs that diet-only interventions work for some people. I literally know people who have lost large amounts of weight and kept it off for years that way. Medications have risks and side effects, so should only be prescribed if behavior modification has failed.
What I did say is that we are increasingly learning that overeating is a very complex and life-threatening medical concern that often cannot be solved through dietary intervention. Even people who may die if they don't adhere to a diet frequently have difficulty adhering to one. Saying that there are "existence proofs" that diets sometimes they work is a terrible response.
My view is that we should treat diet as one possible medical treatment, and evaluate it apples-to-apples (in terms of efficacy and side effects) against more recent FDA-approved treatments. And now that we've seen how much impact those treatments can have -- and particularly how critical the GLP-1 mechanism is to the sustainability of weight-loss interventions -- we should work our asses off to find more and better treatments that treat overeating as a health condition rather than a question of mind-over-matter.
You seem extremely attached to your position, to the point where defending your position required you to declare that my relatives "were eating a bunch of other stuff when you weren't looking."