Help understanding what RS232 vs TTL option means on ultrasound sensor...?
Sounds a lot like "poor mans RS232", which is just an inverterd TTL signal. This violates the RS232 spec (which mandates signal levels above 5V or below -5V using a line driver) but works with a surprisingly large amount of RS232 receiver chips.

I suggest using an oscilloscope to check the signal. It might be just a mistake in the datasheet.

Note that if you wanted to connect the sensor directly to a microcontroller you'd use the TTL option.