Bilstein is referring to the lift over stock Tremor height (which they communicate as "Fits Vehicles with 0" Rear Lift")
Change the vehicle you entered in the Bilstein site (for example Lariat F150 4x4) and it will change to 0-1" lift.
There is another thread on here about this topic that I commented in, but if you are really really concerned over droop travel, then you want to look at the Jeep line of 8112s. They are a little longer than the F150. The valving will most definitely be different though and you'll need to have someone rebuild them right off the bat.