Gasoline isn't explosive. The residual vapor is. Regardless, think he is referring to diesel when he said "gas".
I thought "gas" was strange, but he repeated that 4 times. I certainly wouldn't confuse the two. He said fuel once, but I take gasoline to be a fuel in the generic sense.
I've seen empty diesel tanks explode due to the heated vapors created by a saber saw or hole saw (two separate events). I've seen the aftermath of two separate diesel tank explosions cause by inadvertent contamination with a few percent gasoline (very common when tankers switch load). It doesn't take as much as you think. In all three cases the people were very lucky. They were thrown a distance but not killed. I've seen a shop vac catch fire when used to vacuum diesel from a small tank.
You can roll the dice, but these are not safe practices and are not permitted under OSHA or by any code.
How would I clean the tank? Power washer and manual diaphragm pump is safe. AFTER the tank is clean you can add an access hole for future cleanings, but the tank must be cleaned first. Filling with water is another option, but if the tank is not spotless, you now have 50 gallons of oily water to manage.