Rubber isn't known to "absorb" electricity. It's known to do the opposite. Rubber isn't a connductor, and so it blocks and deflects electricity, which is why it can protect you if there's enough of it.
The sound heard was not thunder. It was the impact of the lightning with the ground.
The bystanders weren't electrocuted because the lightning had entered the ground and, given that this is a city, probably continued via an underground circuit of pipes.
Whether the umbrella was or was not fried depends entirely on the completion of the circuit. If all of the lightning managed to pass through the umbrella into the man before the umbrella was moved, no harm would come to the tool. This is the same reason that the clothes are not obliterated in lightning victims.
