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Measurements under load would be more meaningful but I don't have suitable high-wattage resistors on hand at the moment.
Most modern electronics will require a measurement under load. Most small loads do (a simple 1K resistor). Between protection circuits, and pulse-width modulation, it's rare to see a device that you can measure straight up, nowadays. For a more accurate measurement, use an oscilloscope.
I believe that most pass-throughs on the market, have no circuit whatsoever. This one definitely has "something". Also, I understood that the G6 battery was regulated, no? (regulated; circuit that maintains one fixed voltage, regardless of the battery's charge level).
As for using a computer's USB port, if it is overloaded, some devices will just stop working, so if your USB pass-through disables your keyboard and/or mouse, you know to use another source of power
Unregulated/circuit-less pass-throughs do require a "high power" or "charging" adapter; there's just not enough power available from a computer's USB port to run one.