Not unreasonable?
The volume of air may matter less than the proportion that flows freely vs being blocked by the wheels. The idea of the central jet seems to be that it delivers energized air to the diffuser at the rear.
Air that is parted to the sides self-cancels (on average). Air that goes over the top contributes to lift.
Here's an example from history — a feature that didn't make it from Buckminster Fuller's Omni-directional Trasport, to its ground-taxiing test form, the Dymaxion
car:
He called it an 'air keel'. It was a central jet on a tricycle that fed air to the empennage.