It still takes time for the evaporation to happen, which is why they were at pains to get that drained and lit as quickly as possible. Boiling water turns to steam, but does a pot of boiling water turn to steam all at once? (They apparently dug the ditches in place and used shaped charges to puncture the tank walls. Easy drainage if the space above the liquid was filled with compressed gas.)
It still takes time for the evaporation to happen, which is why they were at pains to get that drained and lit as quickly as possible. Boiling water turns to steam, but does a pot of boiling water turn to steam all at once? (They apparently dug the ditches in place and used shaped charges to puncture the tank walls. Easy drainage if the space above the liquid was filled with compressed gas.)