Thanks for the compliments.
I reckoned since we're talking about use of several seconds only, that despite drag greater than one generated by 90 degree GF, this would add a lot of DF. Part of the air would hit endplate, and the rest that doesn't overflow the flap would spill to the car center-line since there is no endplate.
Perhaps they could make a slit in it (make it longitudinally hollow) to condition the flow more appropriately, or move it backwards, so that only small bit of it faces the upwash stream.
EDIT: What if it would be like on first pic on lower set? Not reaching fully so that wing and flap meet, but having space between at max position?
Remaining two are visual out-loud thinking - in the middle is virtual 90 degree version achieved by longitudinal cut, and the one on the bottom is also intended to achieve approximate 90 degree effect.