consumer fireworks (1.4g) cannot blow up like this, by design and regulation. fire marshals can and do enforce this at points of sale & distribution
professional fireworks are also regulated to minimize mass-explosion risk.
if fireworks were involved in the explosion, it would necessarily be an improvised device. achieving this level of explosion with consumer fireworks would be difficult. the fireworks are a red herring, imo.
According to this article, there was also "gas tanks and camping fuel" inside the truck.