You are on the right track. If the voltage is 120 volts and the device is supposed to trip @ 4-6 mA of fault current, that means they put no higher than a 30K resistor between the hot and ground. (120 / .004 = 30,000) They don't put a "short" between the two, but rather a test load in the range of what the device is supposed to respond to.
One thing that I have heard of that you need to watch for when there is no EGC, and the best part of this is when an electrical inspector is the one telling the story of what happened to him at one time. He encountered an outdoor GFCI receptacle one time and his tester showed no equipment ground. He then pressed the test button on his tester and got a pretty good jolt from the metal weatherproof cover. That is because that tester is trying to put a test current on the non existing EGC and therefore the cover became 120 volts to ground and standing outside is generally much more conductive than inside on a wood or other non conductive floor. So when you see there is no EGC you really need to think about what you may energize before you press the test button.