There seemed to be all sorts of discussion about everything other than the simple math behind weight loss.

Well it seems there's a lot more to weight loss than simple math. I should add that from an outside perspective, there is a pretty good chance you will re-gain the weight you lost.

Anyway, I agree with you that the discourse about obesity/weight loss/etc. is not much better here than in other places. I think part of the problem is that low-carb dieting is almost like a religious issue for some people. I suspect another part of the problem is that self-deception seems to play a role in obesity/overweight. Last, there are a large number of poor thinkers who post here (as in most places); arguably a subject like weight loss which is very easy to have an opinion on is a magnet for them.

Have you ever lost confidence in LW after a similar experience?

Somewhat, yes. I wrote a blog post called "Is Lesswrong More Wrong" a while back.