Do you have a meter so you can independently measure the battery?
EDIT: I realized there is an error in the documentation. The ADC measures 0V - 3.6V, not 0V - 3.3V as it states in the doc. I will fix this.
So the measurement you are seeing is (751 / 1023) * 3.6 = 2.64V. That seems low, but I have seen coin cell batteries operate down to about 2.5V, so it could be possible. The best way to verify is to measure the battery with a separate meter.