You're talking about something that just arc'ed a mile or more. An inch of rubber is nothing. Cars get struck by lightning on a routine basis and they have MUCH thicker rubber tires. The reason a car protects you is that the current travels through the metal body acting as a Faraday cage. The reason he got struck could have been something as simple as riding over a manhole cover or some other item that would tend to provide a good ground. Air flow over a plastic helmet shell may also generate a moderate amount of static charge, especially if its not wet, making your head a prime target.

I remember watching a documentary on lightning, where they were launching model rockets to create strikes. The lightning followed the rocket trail to the ground, then went underground ~10m sideways until it found a good "ground", an underground metal utility conduit. The path was easy to trace because the dirt had been turned into glass, then burned a hole in the conduit before dissipating into the conductor. So even after a bolt hits the ground, it doesn't dissipate instantly.