I’m underweight myself and eat very little (...) I feel chubby after a few days of binge eating

I see, so you are probably affected by an eating disorder yourself and therefore biased. That explains your stance on the topic.

your advice is (...) making the issue out to be much larger than it probably is. (...) OP never used these words

OP sounds inexperienced and probably is confronted with this kind of disorder for the first time. So naturally OP describes it without using certain vocabulary and therefore wrote: "She gained some weight but she is not fat at all!" and "She was underweight before, because she hardly ate anything".

simultaneously suggesting he ignore the problem

So you agree there is a problem? I never suggested to ignore the problem. I only suggested to not join in on the topic of "I am fat", brought up by someone with a possible eating disorder. See next point for reason.

as someone reaching out for help is not doing so to put you in a bad position

Saying "I am fat" when being just above underweight, does not equal asking for help. It equals asking for confirmation of a distorted body awareness and self image.

Blocked to prevent further animosities.