Wait a minute, a lot of those transistors are switching the same currents since they are in series. Also, FETs only draw most current while switching, so in between switches there's almost no flow of electrons. So in fact you cannot calculate things that way.
Yes, as I said the parallel assumption is not quite realistic, and the number is an average, covering all states a transistor may be in. So it amounts to a rough lower bound for when a transistor is switching.
