I suppose they also have a very active d-vitamin production to bind that calcium (since otherwise much of it is wasted, if I remember right), and are more resistent to the negative effects of calcium? From what I remember, an overdose of calcium can cause kidney failure and other sorts of stuff. I also hope the people drinking this milk don't do it as much as we do. :V Or maybe the residents have also become more resilient to the effects/developed metabolisms where more of the calcium is removed from the body -> they are dependent on miltank milk to get the necessary level of calcium.
I like this series, the character of Baobab is delightfully optimistic about life, and you can easily sympathetize with the joy of discovery he has~