Given the huge spread of the debris, it must have been a decent sized boom, no? I mean that's got to be 10's of miles wide in this video.
the flight termination system is sort of a shaped charge that's designed to rupture the oxidizer and fuel tanks. Even if only a few % fuel remains, it'll be a big boom.
do we know when this video was taken? this could just be ship breaking up during re-entry because it lost altitude control. not necessarily the moment of the primary failure.