"But lactase persistence also took root in sunny Spain, casting vitamin D's role into doubt."

Well, Spain is not entirely sunny. Certainly the north of Spain is not. You can see a darker line in the map that divides Spain. This is divided by a set of mountains called "Cordillera cantábrica" that makes the north way more rainy than the rest of Spain.

There are places in Spain where it rains more than in the north, but only in a very small period of the year. Most of the year is sunny there(the part that is not the north).