Counting frames on YouTube, I get about 0.3s for the blast front to reach the top of the 600ft towers. That gives an estimate of around 600 tons TNT so definitely nowhere near all the fuel exploding.
Everyone can be glad though that no hypergolics are involved at least!
The energy of the detonation wave in rocket explosions is typically 1-2% of the energy in the fuel, at least that is the ballpark of what people use for estimating the effects of mishaps.
We also do not know if the tanks were fully filled -- it the past, rocket companies have called 10 second static fire tests a "full duration static fire test." We will probably find out later what it actually was meant to be.
The question was whether during this test the stage was loaded with the same amount of fuel as for an actual flight, or only a small fraction of that.
Well, only a little bit - they do use hypergolic ignition.