I think they mean with regular vacuuming. A traditional bag vacuum can lose suction power from the start of the vacuuming session to the end.

You'll also lose suction power if you unplug the power cord, clog the wand/hose, disconnect the hose, burn the motor, fill the vacuum with water, etc. They can't account for everything out of the ordinary, but cleaning a filter is considered basic maintenance, though it doesn't need to be done every time you use the machine or even every week.