No, I assume they mean that the spacecraft will be down to 1.15kg by the time it reaches Mercury from all the bits it keeps dropping off and the fuel it's burned in order to get there.
So the 1st stage weighs something like 190 tonnes + 2 x 277 tonne solid rocket boosters + an upper stage, normally used to get to geostationery transfer orbit with 10 tonnes of propellant. All to get about a 10 tonne payload to GTO. But I assume it'll be different on this mission, using most of the payload weight as fuel in order to get to Mercury. We're talking nearly 700 tonnes of stuff, to get just over 1 tonne to orbit Mercury.