I'd heard that you were supposed to fill a used tank with water before cutting and that you were supposed to leave the water in while working on it, not empty it.
Filling with water and detergent (or steaming) and draining seems useful in removing as much of the combustible residues as possible, but refilling with water would follow, according to what I understood. The point of filling a tank with water was to reduce the combustible internal volume to near zero. No (or little) space for an air/fuel mix, and thus no large internal pressure buildup or explosion possible.
Seems clear that the tank would need an unsealed opening at the top as a steam vent and filler for the water.
If large flame cut holes are needed elsewhere than at the top, alternate filler/vent holes could be pilot drilled with a cordless (not a corded) drill below the water line where needed. These become the filler/vent holes when rotated to the top for cutting the full sized hole with a torch. Prior filler holes would be temporarily plugged to hold water. The tank is always filled with water for any cutting operation and is always worked on at the top, with an adequate sized filler/vent there as well.
Please note, I'm not recommending that anyone do any of this at all, just bringing it up since filling with water and emptying it out was mentioned earlier in the thread, but there was no mention of refilling the tank after. I thought filling with water was standard safety procedure when working on small tanks in any way.
