FWIW it was about hydrogen - the Hindenburg was designed around Helium (and thus didn't have various safeties around hydrogen) but due to embargoes against Nazi Germany they couldn't get the necessarily helium, so they filled it up with hydrogen against the original spec.
Hydrogen is not picky about fuel air mixture; it will explode at any concentration between 4% and 74% (in air). I rewatched the footage and it sure looks like an explosion to me.
https://www.youtube.com/watch?v=OetzoO3Csj4
The thermite paint hypothesis is interesting but a bunch of hydrogen airships exploded. The Hindenburg was partly made from metal recovered from the R101. The R101 exploded on her maiden voyage.
At 24 seconds in, that looks indeed fast. But still not what one would expect of "Knallgas" going boom.
Yes. But still the paint burned first. And the hydrogen didn't explode, there was no "Knallgas". Even in all that chaos, the opportunity to mix in the right ratio with air to enable that, didn't arise. It just flared off.
One could even argue that all that flaring off generated some lift by updraft, making it crash softer, more slowly.