ITT lots of people who (unintentionally) under eat.
Having been on both sides of the spectrum (obese ~85kg at 14, then full-blown anorexia) it is fair to say that fat and skinny shaming exist and in my experience can actually be harmful. Granted I had a mental illness, which probably made things much worse, but skinny shaming just made me annoyed (because I often disagreed) and would motivate me to become even skinnier (down to 45kg about 5 years ago, age 16). Vicious cycle.
Now I'm at a healthy weight (82kg at 5'11) with a fairly muscular frame having been lifting for a couple of years and all the shaming comments have dissipated.
Something that I notice as an ex-ED sufferer is that in the vast majority of cases, nobody knows how much they are actually eating on a day-to-day basis. For me it's second nature to have a ballpark estimate, even if I'm not counting, but most people just seem to have no idea – and if they do think they have an idea, then after running through it with them they're almost always miles off. This is why we have people who claim to "not eat much" and are obese, and "eat like a pig" but yet are stick thin; in both cases these groups are grossly under/over estimating their caloric intake.