If the plane is in a flat spin, what would happen if you put the flaps down to cause the backward turning wing to be pushed down, and the forward turning wing up, to tip it out of the spin.
The chordwise airspeed at the flaps (mounted about twice as close to the CG than the ailerons, rudder, etc.) would be about half the airspeed felt by those already mushy-nearly-ineffective primary control surfaces. So the roll moment imparted by the flaps would be so slight that it might take hundreds of rotations, and tens of thousands of feet of falling, before significant bank developed. And then the primary controls are still as mushy as before, so you're no closer to recovering. See Is it possible to recover from a flat spin?