LP in a tank is under fairly high pressure (which varies significantly with the temperature), and this is quite unlike any other "normal" liquid fuel that I'm aware of.
Maybe the misunderstanding was because the Generac's are advertised as (spelled out) Liquid petroleum or Natural gas in the description. How would a device that actually ran on the liquid form receive it? I suspect it would need to be pumped or gravity fed like any other liquid fuel.
I don't know about the pump, but when I went to several sites reading up on this, evidently there's a device called a "vaporizer" (the closest analog would be a carburetor in a gasoline engine) that converts the liquid fuel to vapor, using the heat of the engine's coolant to accomplish this. It seems to be for large engines in mobile equipment, where having a large tank to accomplish the vaporization in the empty space above the fuel would be impractical.