I would not be too surprised if this ship carried a legacy ammunition outfit from Soviet era that is long past it’s safe storage period. The fire may have resulted from attempt to fire over age ammunition, possibly a solid rocket boosters on one of the P-500 missiles exploded when the missile was attempted to be fired.
If the Russians were truthful about there being no casualties, that suggest the ship was at a high state of readiness and the crew were at their action stations away from the big missile tubes when the accident occurred.
It would be difficult to imagine there being no casualties if the ship was hit by 2 missiles, or if there was extensive fire fighting afterwards. I suspect the fire was amongst the cluster of P-500 missile tubes in the front, and resulted from a lunch misfire. The crew then quickly abandoned ship because the threat of other missiles cooking off. So there was no causalities. The crew may have reboarded the ship after the fire went out without setting off more missiles.