This answer is not correct. Remember that the S_N2 mechanism involves a backside attack of the nucleophile on the carbon atom attached to the leaving group with inversion of the groups attached to this carbon atom. In order for the substitution to happen in this case, the nucleophile would have to somehow get inside the cage of the bicyclic ring and then invert the cage through itself...yikes. The following space-filling model is a bit hard to visualize but it does show that there is just not any room for a nucleophile to enter the cage.