Lifts can be made for most widths from 10' to ??'. The one in the ad is a "side tie" lift that does not actually need a slip. It is designed to be attached on one side only, and actually has bulkheads in the tanks making 4 independent air chambers. The white parts above each tank are styrofoam floats that are attached to the tanks with 3-4' arms, and keep the lift from going any deeper. These are exclusive to the side tie type lifts. When raising a boat on this type, you have to tie the boat to the lift with a spring line on each side, inflate the front air chamber on each tank, raising the bow up. The spring lines hold the boat on the lift during this time. Then you inflate the rear chambers and top off the front afterwards. Each chamber has it's own valve, and YOU have to keep it adjusted to make the raising and lowering even. Most side-tie lifts are quite wide, often 14 or more feet, to give the lift a stable "foot print" in the water. Side-tie lifts can work in a slip in the above manner, but they lack the normal attachment arms of conventional lifts, and I'd expect it to be expensive to retro fit them.
Conventional lifts are attached to both sides of the slip with 4 or more arms that only allow the lift to raise in a flat arc. The dock simple provides stable attachment points. these usually only have one valve, and the air stabilizes itself between the two tanks because the arms and the dock prevent any twisting or racking of the lift.