They're not fully sealed. There are two shafts which connect the King's chamber to the exterior of the pyramid. The lower ritual congregation area is not fully sealed off from the upper chambers either. Which means bats are a constant problem in pyramids.

You could easily get a fire going inside one.

Fortunately, it actually is somewhat challenging to get a fire going inside a bat.

Famously, their feces are not at all challenging to set on fire.

Still on the outside of the bats ;-)

When a catastrophic event is catastrophic enough, your other problems cease to be your problem.