I agree with your overall message that "most modern diet advice comes across as more insulting than helpful" (at least that's how I interpret it), but this reply isn't doing it for me.

eating habits have very little to do with weight

Source? "Calories in, calories out" gets repeated to the point of redundancy during conversations like this, but it's really all there is to it. If you consume more calories than you burn, you'll gain weight, and vice-versa. The exceptions to this rule are so rare and medically-anomalous they really don't fit into such a broad conversation as this.

And being overlyskinny or dehydrated to the point where you have no fat and can see muscle is more unhealthy than being out of shape and fat.

Again, source? This is also the extreme opposite spectrum. No one (worth listening to) is saying that this is preferable to obesity. What you call "fatphobia" is only so common now because obesity and obesity-linked pathologies is the most common cause of death in America. It makes sense that more people would be focusing on curbing these deaths than focusing on something that, speaking candidly, affects a much smaller percentage of the population.