Except he really should have metered all the lights to be the same intensity (softbox too), like at a normal portrait subject distance. It would be comparable then.
A shoot-through umbrella puts 2/3 of the light out the back (shown well), so a reflected umbrella is naturally twice brighter. This entirely accounts for his alleged "hot spot", not any property of the light. If the lights were equal at the subject, it is the shoot-through that would have a hot spot (in center of pattern).