The fallacy of the plugin hybrid MPG

I saw an article the other day that set off my peeve alarm when it discussed a plugin hybrid that got 100 MPG.  Now most everyone knows that MPG is Miles Per Gallon.  That means that for every gallon, you get 100 miles.  But since this is a plugin hybrid, many of those 100 miles are coming from the battery being charged from the grid, not the gallon – so its just not fair to count those miles in a MPG rating.  There are some ways to count kwH as gas equivalents – but no attempt was made to do this.  Again, the key point is that in a regular hybrid, the only energy input is the gas.  So MPG = MPG.  But with a plugin there is another energy source, so you can’t calculate a simple MPG from Miles traveled per gallons used.  Put another way, if I carried around a gallon of gas in an all electric vehicle – would that count as an infinite MPG car?