The funny thing is I am fairly sure most Doctors would suggest that being obese does mean unhealthy. It means increased risk of heart disease, joint problems and diabetes for starters. Losing weight through regular exercise is almost certainly making Steph healthier.
Pretty much. Sure, you might get the odd person who manages to be healthy despite being obese, but there are no guarantees that they won't develop problems in the future, and more important, those people are not representative of the overall population.
Edit: Also, BMI is only bullshit when used improperly. It shouldn't be seen as the be all, end all of health, but when treated as a tool to roughly estimate the risks factors for an average member of the population (i.e., not a body builder or extremely tall/short) and combined with other stats and a person's lifestyle, it's quite useful for preventative medicine. Likewise, being overly rigid about the "normal" range and assuming that anyone who falls even slightly outside of it is definitively less healthy than those who are within it is rather absurd, but concern for major deviations (e.g., obesity) is completely justified.