I have reasonable doubt.

All but one containment building had already blown up from hydrogen buildup inside. Now how could hydrogen explode like that in the open air?

It would have had to have been confined or in a vessel under tremendous pressure. PErhaps the cooling pools still had roofs and walls and the explosion came from the cooling pool containment area from a hydrogen explosion but that would have released vast quantities of nuclear materials from the pond.
In which case it could have been hydrogen but along with it the cooling pond and its contents...a very bad thing akin to a dirty bomb.