No, it isn't a blatant lie. The lavatory has two different flush mechanisms. One operates via a pressure differential when the aircraft is at altitude and there's another flush mechanism that operates via a vacuum system for when the aircraft is on the ground or at lower altitudes. When the vacuum system is INOP, the lav can't be flushed on the ground or at lower altitudes with a smaller pressure differential and so the lav use is restricted until the aircraft reaches an altitude where the pressure differential system can be used.