As the linked site explains, this is not about aerodynamic drag of the flap but the whole drag of the vehicle.
F1 cars use rear wings to increase downforce so they can turn faster. The downforce of those wings increases the load on the tires and, consequently, the amount of side force they can tolerate before they skid. This allows to drive faster through turns.
This tire load, however, increases rolling resistance, so on straights it would be better to do without this downforce. In airplanes we use moveable flaps but the F1 cars have decided to vary flap lift by blowing air into the low-pressure side. This raises local pressure, reduces downforce and, consequently, rolling resistance so the car can drive faster.
In effect, by switching the air flow on in straights and blocking it in turns, the car can be optimized to both run faster on straights and run faster in turns without skidding.
Since the low-pressure side of the wing and flap is pointing backwards, increasing local pressure also should reduce the amount of aerodynamic drag there. But the main effect is to vary the lift this wing and flap produce and thus to vary downforce such that the car can reach higher speeds overall.
Stall in aeroplanes means drag goes up because lift needs to be produced and stalling makes this less efficient. Here, however, stall will greatly reduce lift - in cars there is no need to keep lift equal to weight! And reducing lift will also reduce drag, even though the L/D ratio might go down in the process.