I think you and Doug are talking about different devices. Doug's talking about a cooling system pressure tester, that pressurizes the system. You're talking about a vacuum coolant system filling device, that pulls a vacuum in the system, then uses that vacuum to draw coolant into the system.
You're correct in that the vacuum filler completely eliminates any chance of air in the system, as long as there are no leaks. The first time I used mine, I left the system under vacuum overnight before filling it, so I then knew without a doubt, there were no leaks.