If the combined mass is greater than a billion suns, why is the energy only worth 100 million supernovae? Is only 1 mass in 10 gonna blow?
That reason is going to be buried in the equations. Remember, this isn’t the explosion of a billion suns, it’s the merger of two black holes having the mass of a billion suns. And the resulting gravitational hiccup will have the energy of 100 million supernovae. Other than that, I can’t tell you much, not knowing the mathematics.
Interesting that a significant percentage of the total mass involved will be lost to producing a gravitational wave.
All my mass could be converted to a gravitational wave and they probably couldn’t detect it even if it happened right next to the LIGO detector (which is a total, perhaps very erroneous, guess on my part!).
OTOH, all my mass could be instantly converted to EM radiation via some nuclear process and, OMG, would they ever be able to detect it, and detect it far, far away... Makes you go, “Hmmm...”