Yeah the surprise of the person when saying "compound that kills only humans" is odd. If it's just the flood, everyone should know by now, yes? But perhaps there's an aspect of the flood perhaps just discovered (or just little understood) that allows a controlled implementation of the flood spores, a human-directed weaponized flood spore? It's curious. But it must have to do with Flood; why introduce something brand new and unrelated to anything previously Halo? Covenant and Flood arcs are still plenty :P