They're saying that it's the inertial mass that is negative (no mention of gravitational mass).
Well they're the same so if one is negative so is the other. Isn't it intuitive why they're the same? Inertial mass is resistance to acceleration... force acceleration mass thingy... well that what's happening in a gravity field... gravity provides the force, the ground causes acceleration in the form of constant change of velocity (constantly stopping you falling at terminal velocity), so gravitational mass is the resistance that stuff has to this acceleration. How has this not been resolved like 70 odd years ago? Why is something that causes brilliant people headaches so obvious to me? Is my weed really that good or am I missing something?