Equal treatment of a set of groups does not suppose that the groups are equally good at the skill. Rather, it means that every individual will be assessed individually, without consideration of which groups they belong to.
Banning thin models from the runway is wrong, in so many ways. It sends the message that all thin people are unhealthy, which is not the case. It is as wrong as it would be to ban fat models. The intent behind banning thin models is good, it is the execution that is problematic.
At the most, it would be okay to set a quota for thin models. Say, require that no more than 20% of the models be outside the typical BMI range (18.5 to 25), or something like that.
Wanting to be thin is not a problem in itself. But if society pressurizes people to be thin, then it is a problem.
I have a suggestion to fight unhealthy social pressure: decentralize norms of attractiveness. Instead of saying “She is beautiful”, say “I find her beautiful”, or “She is stereotypically beautiful”, or something like that. Similarly for other genders and for gender-neutral language.