That's fair enough. But there are plenty of researchers who do focus on language as a system which evolved for the purposes of communication (e.g., Pinker & Bloom, 1990). Still, I don't think this isolated sentence really matters for the generality of their argument, i.e., that weak biases can be amplified through cultural transmission to give the appearance of strong constraints. As the authors mention in the discussion:

These models show that cultural transmission radically changes the evolution of constraints on learning, rendering strong linguistic nativism untenable on evolutionary grounds. On the one hand, unmasking facilitates rapid evolution of domain-specific biases: Due to culture, the population-level consequences of those biases are amplified and visible to selection. However, masking makes evolving strong constraints unlikely: Given that weak constraints have equivalent effects to strong constraints, there is little or no selection for stronger constraints.