AH276 hall effect sensor with microcontroller (sensor already in circuit - brushless fan)
The two output pins (2,3) are open collector NPN transistors. These will be switched on and off at a rate that sets the speed of the fan. So your guess is correct.

Coils can produce some spiky voltages so I wouldn't recommend directly connecting these pins to the input of a microcontroller. A simple resistor/transistor should do.