The data analysis showed strong relationships between vocal repertoire size and group size, as well as between repertoire size and the amount of time spent grooming...
What the study did not indicate was causality, in other words they could not say whether one of the three variables caused the other two to develop. Look at it this way. Larger group sizes need more mechanisms to maintain social solidarity (such as grooming or increased vocal repertoire - which could serve to increase group identification). On the other hand, increased vocal repertoire could allow larger groups to exist. Chicken or egg?
One note of caution though:
It is also important to remember that there are radical differences between non-human primate vocal repertoires and human languages, says McComb (one of the researchers - afarensis). So it does not follow that languages as complex as ours will necessarily follow from increases in group sizes and social interactions. “There are other big hurdles that have to be overcome to get to human language,” she says.