Not sure what you're point is, but she runs ultra marathons she is clearly very strong and healthy. Would you say an Olympian is too thin?
Those saying she's too thin really mean she's too thin to be attractive to men. As you say, how can she be too thin? She's probably fitter than the majority on this thread if she's running ultra marathons.
Too thin has nothing to do with health, it's just a guise for 'ew I don't like it' especially when the person is physically fit.