Grill and up is more important for protecting the really important human bits, ideally you want to lessen the impact by making the pedestrian roll over smoothly, it’s why Nissan (and Suzuki, might be others) had hoods that prop up automatically. This is just the logical next step.

edit: completely forgot about Volvo. From the article

Pedestrians 80cm tall and above can be detected, and crashes can be avoided through full automatic braking at vehicle speeds up to 35km/h.

So it seems this airbag is intended for faster crashes.