You are correct. That (extra) length of the magazine exists to accommodate the 10-round capacity that has become the popular standard for .22LR semi-autos.
Therefore, a flat/flush floorplate is not possible unless the overall shell of the magazine were to be shortened, thus cutting back the capacity to 8 rounds (which I believe is the original capacity of the vintage PPK/S .22 LR).
Personally, I actually wouldn't mind an 8-round mag if it were to sport a flat floorplate.