It would not be racist due to it being wholly on medical reasons. If they wouldn\'t let in someone who had some sort of waterbreathing magical device, however, then it becomes racist.
Think of it this way: If a hyper-tall Blue tribe would only let in hyper-tall people, and the Red tribe was all short people, it wouldn\'t be racist IF, should, on a fluke, a Red tribe member be tall enough to join, they would happily let them in, and similarly, a Blue person being less than tall enough, would be kicked out of the tribe.
If there was a purely social reason for this, it would be some other sort of bigotry, but if it was a survival thing, than it would just be incidental.
If only tall people can survive in the Blue territory, it\'s not their bloody fault (unless, of course, they moved there solely to get away from short people).