Both mAh and Coulombs are measures of charge (hence energy) storage. There is no consideration of voltage, just a measure of how much energy the capacitor can store when fully charged. It doesn't matter how the voltage varies from 2.7V to 0, linear, non-linear, etc.

Think about a 5.4L tank of water. The capacity is 5.4L. If you drain it at 1.5mL per second, it will take 1 hour to fully drain it. Or you can drain it at 3mL per second for half an hour. Or start, stop, start, stop, etc. It doesn't matter -- the capacity of the tank is 5.4L.

Same for the capacitor. The capacity is 5.4 Coulombs of charge. If you drain it at 1.5mA it will take 1 hour to fully drain -- hence you can say it's equivalent to 1.5mAh of capacity. Any other way you do it (3mA, linear, non-linear, etc.) does not change how much charge (hence energy) the capacitor can store.

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