This is correct. It won't be a sprinkler system because the two most common types of fire will be fuel or electrical and water doesn't fix that.
My guess is the engine room had some C02 system to put out a fire but it only works if its both maintained, and the door is closed so the c02 can do its work.
Galley fire at 3am when everyone's asleep? Unless the cook was slow roasting a roux or something, seems unlikely. Galley fires happen when people are in the galley cooking.
My guess is that this is either an electrical fire or some dumbassery like someone left a bright light close to and shining on something flammable. Fuel fire would require first a fuel leak, which would smell like diesel and make everyone sick, before it ignited. Fire spread quickly on main deck and by the time people on lower deck realized there was a fire, their only exit looked like hell's entrance.
A decent fire detection system prevents things like this but either they didn't have one, didn't maintain it, or something.