The rig still displaces the same amount of air, but with a reduction in drag. If anything a tractor-trailer drag reduction benefits
other vehicles in adjoining or oncoming lanes.
The side skirts are only a partial solution compared to enclosing the gap between tractor & trailer, the next real benefit will come with retrofit "kammback" devices at the trailer doors.
Trailers designed from scratch for best aero performance, as with this rendering of a UK Don-Bur design:
When air is kept from crossing underneath the trailer, or between tractor & trailer, there is a reduction in drag that is in all ways good.