Any system that is open to atmosphere has moisture in it simply because all air has water in it. That is the relative humidity. The world's lowest recorded relative humidity value occurred at Coober Pedy in the South Australia desert when the temperature was 93 degrees and the dew point was minus 21 degrees producing a relative humidity of 1 percent. The average Phoenix humidity is between 35% to 40% according to my quick Google research.
I don't know what the volume of air is in a 911 system. With all the hoses and parts is is a decent amount however. My vacuum pump runs for a few minutes before it starts to struggle.
Yes, it is physically possible to charge the system without pulling a vacuum but it would be a waste of time and refrigerant. It is physically possible to charge it with propane but I sure would not want to try.
Within just one second of being open to the atmosphere it has pulled air to every part of the system. Every bit of that air has moisture in it, even if you live in Coober Pedy. 100% of the air has no value as a refrigerant and only messes up the AC system.