As I see it, the secondary gap (that introduced at the distributor) allows the coil voltage to build up to a high level ~30-45k volts before the arc is initiated across the secondary gap. Once the arc is initiated the voltage drop across that gap would be 3-5k volts. The excess 25-40k volts now appears across the spark plug gap. The delta V/delta t is rich in RF components that are very effective in ionizing the air between the capacitive gap. With the ionization of the air, the resulting arc can now be initiated across the gap. The current leakage of the fouled spark plug still exists but it is not as effective in reducing the developed coil voltage once the arc is extant across the gap.
If you have ever worked with a Tesla coil or the flyback transformer of a TV, you will experience the ability of RF ionize air. In fact the flyback horizontal sweep circuit is a lot like the inductive discharge ignition system. The rectified HV can reach 30k VDC to provide the high voltage for the picture tube.
I once had a B&S lawn mower engine that would no run unless the secondary gap was introduced between the spark plug wire and the spark plug center electrode. Even scraping, cleaning, and drying the spark plug did not cure the problem. Only installing a new plug cured the misfire.