According to these people, anybody who doesn't eat whatever they're immediately craving, as soon as they're craving it, and in whatever quantities they crave has an eating disorder.
You're concerned about food quality? You want to make nutritionally informed food choices? You prefer to avoid ultra-processed foods, refined carbohydrates, and added sugars? It doesn't matter. None of it does. If you exercise any restraint over what you choose to eat, you've got an eating disorder, because you're fatphobic, which also means you're racist. Oh, and you're also ableist, because you don't want to end up afflicted with any of the myriad health conditions caused by obesity and poor diet (except, according to the FA cultists, they totally aren't).
You cannot win. You cannot satisfy these people (any more than they are able so satisfy their own insatiable appetites). You cannot reason with them. There is no point in being a thin (or smallfat) ally to politicized deathfats, because they will still resent you, hold you in contempt, lecture you about the deference and accommodations you owe them—and cancel the fuck out of you at the first opportunity.
Seriously, the best course of action is to relentlessly mock their shitty ideas, while leaving them to self-destruct for the cause. Maybe if enough of them ruin themselves with food to the point of premature aging, debility, and/or death, potential new converts to the cult might just start to catch on.
(Forgive the rant; I'm in that kind of mood tonight.)