If they use 70+% (or even less) it will certainly react when it is only in ordinary bottles and O2 will be produced very fast, I guess that the bottle will bubble a lot and there will be a lot of H2O2 all over the place as well as burns for the person that opens the bottle. A nice countermeasure would be to use ultrasound and agitate a possible H2O2 bottle, it will for sure "explode" in a harmless explosion.
The bottles containing the liquids were to have false bottoms.
I imagine the terrorists were planning a trip to the privy for their chemistry. I suspect they had each tried this process on the ground so they knew what to expect.