[Sneaky Weasel]

The Gods most certainly are using homebrew, and if you'll recall, Hephaestus' aura is divine fire and not subject to resistance. He also has a plethora of fire based powers able to pierce immunity, and he might have used some, if he hadn't been beaten so fast. The lava stems from his death, similar to a Balor's death throes. Basically, it's coincidence that this is regular fire damage, because it's not like Hephaestus was planning to blow up. He expected to be blowing up others, and prepared for that eventuality.