Another view
Let's look at "it" another way
More drawin's
...& some more expanded explain-ation(s)

Havin' the front of the racer "blocked off"
...or not
...isn't a big deal, while just sittin' still

When driving along
...& having the front of the racer blocked off
...a lot less air would flow under the racer
...but, IIUC (if I understand correctly) a high(er) pressure area (drag) would be created in front of the racer
...& the faster the racer "goes' the higher that pressure (drag) would get
&/also
As a "Free Bonus" a low(er) pressure area (another form of drag) forms behind the racer
& (remember)
The faster the racer "goes" the higher the pressure (more drag) that is exerted, on the front of the racer
...& /also, (in turn) an even lower pressure forms behind the racer (even more drag)

But, think of this...
...even "if" the "fancy" shaped floorboard, doesn't actually create any noticeable down force
...the undercar air flow, along with the Venturi effect, should still "inject" some high(er) pressure air
...into the "known to be created" low pressure, area behind the racer
* So, to re-interate
...it seems like having air flowing under the racer
...& thru some venturi tunnels, would help reduce the drag, on the racer, in (2) different ways
1.) From the front, of the racer
...by allowing the higher pressure air (that has built up)
...to "vent" under the body
&
2.) By injecting that "manipulated/conditioned" higher pressure air
...into the lower pressure area, behind the racer
...should help reduce drag too
