It has to do with stored kinetic energy, look at it this way , if you fill it up with pressured air , say 3-4 pounds pressure and it splits then you are dealing with 20 gal stored air with immense energy,a split could rip with explosive forse. If you fill up the tank with water to say 19.95 gal and then apply air pressure to the tank you will only be dealing with a air volume of .05 gal at a pressure of 3-6 lbs. of pressure, a split in the tank will only result in water with a quart of air coming out. The energy stored is one quart in volume not 20 gal of air volume. a spirt of water vs. a explosive blast.
One could run a mole calculation and figure out the total energy difference if you are that conserned.
If it was me , and I have to do the same thing since I will be adding some bung holes for fittings to my tanks , I will be using water with air to pressurize. You can pressurize with 3-5 lbs of air and use a slap dolly or a broad hammer on a seam and the little bit of pressure will force the crease out.
Harley uses water pressure to form their modern bike frames except they use water under a couple of thousand lbs of pressure on thick steel. Their process starts with semi folded steel parts that have been bent to allow them to be placed in the form then they are pressurized, you do not even see the original folds or bends after the pressurization.
Plus side , it just costs the price of the water and a little electricity to pump up the air compressor and a couple of fittings . Like pumping up a tire.