In practice, wherever I have encountered 'Fat acceptance' I have encountered science denialism.
'Being overweight and sedentary doesn't cause diabetes.'
'Obesity doesn't contribute to heart attack and stroke.'
'You're wheezing because you have asthma, not because you are 400lbs and fat is literally blocking your windpipe.'
As some examples.
I'm all for not being a dick to people.
Science denialism needs to be stamped out with a fucking vengeance. Actions have consequences and lying to (young people) about the consequences of their dietary choices helps exactly nobody.