The spitter bolt bounced off the cable in the ceiling to hit a propane tank in the darkness. Which then proceeded to leak and blow up exactly as Cirno was entering the room. Stuff like this is exactly why the death replay is so great in Noita, stuff just happens so quickly you need a frame-by-frame analysis.
I'm pretty sure what's going on here is that in addition to the fire and explosion damage, they were also dealing projectile damage, which is its own damage type. As you were not immune to projectile damage, they were still quite capable of ending your life. :)