The magneto-optical Kerr effect (MOKE) is often assumed to be proportional to the magnetization of a magnetically ordered metallic sample; in metallic ferrimagnets with chemically distinct sublattices, such as rare-earth transition-metal alloys, it depends on the difference between the sublattice contributions. Here we show that in a highly spin polarized, fully compensated ferrimagnet, where the sublattices are chemically similar, a signal is observed even when the net moment is negligible. We analyze the spectral ellipsometry and MOKE of Mn2Rux Ga and show that this behavior is due to a highly spin-polarized conduction band dominated by one of the two manganese sublattices (4c) which creates helicity-dependent reflectivity dominated by a broad Drude tail. Our findings open prospects for studying spin dynamics in the infrared.