OK - there is a lot to unpack here . . .
First, the pressurized air system of our air guns is extremely different than that of a refrigeration system, with the most important difference being that a refrigeration system is a "closed" system and the air gun system is an "open" system.
In refrigeration, we need to get all the moisture out for it to work properly, and the method for that has already been described. Once we know we have a good sealed system with no moisture in it, we fill it with refrigerant and it does its thing - sealed as a closed system, at least until a leak develops. And then refrigerant leaks out, but moisture typically does not get in, except in cases where there are bad enough issue son the suction side (which is almost always still above ambient pressure when running).
But with the "system" of filling air guns, everything is different due to the open nature of it. Even if everything were to start off with absolutely zero water molecules in it (in both liquid and vapor forms) via whatever method one wanted to use (vacuum, nitrogen purges etc.), it would be possible for moisture to be introduced into it with the very first use of a pump to refill whatever reservoir. When that pumping refill happens, ambient air is drawn into the compressor, and if no action is taken with respect to reducing the amount of water vapor that remains in the air stream exits the compressor, remembering that the air will be hotter than ambient due to compression, some of that vapor will condense into liquid when it cools back down to ambient.
As @Gerry52 said, this is easy to prevent with proper filtration - usually with desiccants. But if no action is taken to actively filter out the water vapor, it is a certainty that over time liquid water will accumulate in the tank or reservoir. The amount introduced is each pumping session is small, but the majority of it stays in the tank or reservoir and more gets added every time the system is opened up on the pressurization side to add more air into it - which has to happen every so often as we are opening the system up on the depressurization side to use air for shooting.
There is more I could add, but that should start to make it clear.
Also, the is no need to pull a vacuum on a tank before it gets filled - as long as there is no liquid water present to start, the small amount of water vapor that exists in the ambient air in the tank is inconsequential - remember that if we are filling the tank with appropriately dried air, that starting level will be dilute 300:1 if filled to 300 bar gauge. But even if you did do that, the tank should have no issue withstanding 1 bar of negative pressure as they are so strong to begin with to withstand 500 bar of positive pressure - they could not handle that kind of negative pressure differential, but 1 bar of negative differential would be noise to our tanks. Remember that that those huge train tanks that are shown collapsing are not made for pressure, but for liquids . . .
Almost correct... Train LP tank cars do the same thing, made for pressure, not vacuum.
This is exasturbated by the 2-3mm aluminum lining inside a CF bottle.
Seriously, have we already forgotten about the CF sub that imploded last year?
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