No it is not because these diseases occur in only a small proportion of those populations. The role of a doctor is to ascertain the cause of a disease and every individual is different. You cannot treat a disease statistically, you have to treat it specifically. If a genetic disease is suspected one has to ascertain whether the genetic indicators are there in the individual. Therefore this study is totally worthless for treatment of disease.
In order to treat "the individual" properly, a diagnosis must be made. In order to do that, a history and physical must be done, along with appropriate laboratory screening tests. Knowing a person's ethnic/genetic background allows for a much more efficient approach to the individual.
As far as your stuff about environment affecting skin color,so what? Of course it does, just like it affects everything else (more or less). Our genes give us a range, our environment puts us in specific point in that range.