Manuacturers are famous for having instruction manuals that are filled with bad grammar. That might be what is happening here.
If a piece of equipment has a metal case, it ought to have a three prong cord. Period.
Apart from safety issues, many electronics use the ground as a 'neutral' for the tiny amount of power necessary to keep things 'warmed up' (for instant 'on') or allow them to function when the appliance is "off" (timers and photocells are some examples where this might happen).
Now, the manufacturer may be confused between a "grounded system" and what you think of as a ground wire.
A "grounded" conductor is what you think of as the "neutral" wire. Not every circuit needs to have a neutral...for example, American 220v devices, such as space heaters, are served by two "hot" wires that are out of time with each other. Contrast this with the typical Euro "220", where there is a 'hot' and a 'neutral.' This might be what the manufacturer is referring to.
Assuming a product was bought in the US, came with a two-prong cord, and has a UL lable, you should be able to plug in without worry.
