volts are volts. the key element is the BEC in the ESC, which will give a constant 6v at something like 1 to 3Amps.
I guess the instructions are relating to its use in a battery powered setup with just receiver and servos, like in a nitro car. different battery chemistries have different voltage outputs. it was probably written before lipos were common.
I don't think you'll have any problems.
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