Yes and no. They don't measure power at all, they measure speed and then use a formula to calculate the power a hypothetical average rider would use to sustain that speed on the road. The formula is pretty consistent over the relatively narrow range you use in threshold training. So, if it underestimates your FTP by 10% at FTP effort it will be very close to underestimating it by 10% at 80% of FTP effort. So, you just use the number it thinks is your FTP and train away.
That said, it's really a waste of money. The formula is available on Kurt Kinetics website. The watt display is instantaneous only, it won't record your work out like a cyclometer will. Just get a cyclometer (if you don't have one) and then use the formula on the Kurt Kinetic website to determine what speed to do your intervals at ahead of time.