Bob describes the battlefield around the Stone Table as a conjunction of the model "place in space", like Halloween is of the model "place in time". This is where Aurora was killed, so she (and any other Immortal killed there) is D-E-D dead.
I don't for a second think that killing the Knight is the only way to remove that particular Mantle. There's been too many hints to the contrary, and Mab has (and had) everything to gain by letting Harry think that's the only way. It's probably ridiculously difficult and most likely requires a conjunction, but I doubt killing is the only way.