That is not "ser" + gerund (which has no meaning), but "ir" + gerund. In the present tense, we would say:
... conforme los pueblos se van dispersando, las lenguas se van diversificando.
Unlike "estar" + gerund, which merely implies that the action was taking place at a given time, "ir" + gerund means that an incremental change was taking place. The difference is usually subtle, and they may even be interchangeable at times. For instance, there is little difference between
Los pueblos se fueron dispersando y las lenguas se fueron diversificando.
Los pueblos se estuvieron dispersando y las lenguas se estuvieron diversificando.
As "dispersarse" and "diversificarse" are incremental changes (they got more and more scattered over time), "ir" + gerund is preferred, but "estar" + gerund can be used, too.
In this case, however:
... conforme los pueblos se fueron dispersando, las lenguas se fueron diversificando.
This means that two different incremental changes were correlated: The peoples were getting more and more scattered over time, and the more scattered they got, the more different the languages became from each other.
As the example is specifically talking about the relationship between incremental changes, "ir" + gerund is mandatory here.