Not knowing the design team I can only speculate their intentions. But in this instance I?d describe that part as an air dam. The shape of the vehicle kinda forces the air dam to become side skirts as well.
There are essentially 2 ways to manage underbody flow; use it and guide it, or prevent it. SD is of the latter camp. Vesco being of the former camp.
Typically the direction to airdam a car is driven by the testing methodology. I?m talking auto industry here. Typically cars developed in a static ground tunnel will benefit more from an airdam. Those developed in moving ground tend to find opportunities under the car not seen in a static ground tunnel.
Neither answer is more correct than the other.
Safety could be a deciding factor in the decision to let air under the car. On a flat bottom shape like SD you have to manage the pitch of the car quite carefully to avoid choking the diffuser and destabilizing the car. Sometimes it?s just smarter to not let it under in the first place.
Again knowing nothing of the design process of the team everything I say is speculation, but educated speculation.