1. Myth: anorexics don't eat. Fact: yes they do, just not enough. Actually I think the range between 1200-2000 kcals can be the trickiest as it still feels like you eat "quite normal" amounts when you're not. I'm not saying that subclinical levels (below 1000 kcal) are any better, but for my part, I realized quite fast that living on such amounts is just not possible and got desperate quite fast because of it.
2. Myth: you need to have lost your period to be sick enough. Fact: it is a good indication if you have lost it, but if you still have it it may not mean anything. You may still be sick enough. Especially if you're on birth control pills, this is not a good indicator as the pills will give you a fake period.
3. Myth: to be an anorexic, you need to have starved yourself for many years. Fact: time is not a factor. If you have negative thoughts around food, do restrictions without no good reason (allergies and intolerance) or have a control issue with food, you have an ED.
4. Myth: you need to be underweight according to the BMI scale to be sick enough. Fact: BMI is a crappy tool and everyone has their own individual set point weight somewhere on the scale. Being below your own set point is in fact underweight.
That's what I can think of right now. Can you think of anything more? Please share