Yes, you are right. I didn't pay much attention.
Measuring frequency with RPi it's a (pretty) hard task. You could use a timer/counter with input set to a GPIO pin (not sure if possible) or you could poll the GPIO (or use interrupts) and determine the frequency. Either way, I would get a different sensor since it's too much trouble. There are lots of I2C/SPI sensors out there.
You could also use some uC (AVR - tiny, mega; PICs) to measure the frequency and output the data in a more useful format for RPi (I2C, serial, etc.).
Another way would be to use an external parallel counter. You connect the output pin of the sensor to the input of the counter and the reset and data pins to the RPi. On the RPi side, when you want to get a read, you reset and enable the counter, wait a period of time (100ms, 500ms, 1sec) and read the value. Knowing the number of impulses in a period of time, you can compute the frequency. Depending on the time period you choose, you need to compute the max count value for the counter (max output frequency is 10KHz).
1 sec means 10000 impulses max, so the counter has to be at least 14 bit (16384).
100 msec means 1000 impulses max, so the counter has to be at least 10 bit (1024).