Note the comment at the end of the paper. The authors had not been able to do it successfully with their relatively simple methodology. Sure it is harder than DDR2 but this doesn't mean it is impossible. As pointed out by the authors, the failure can simply be due to the memory controller implementation (or DDR3 protocol itself) on their test setup. If this is the case, then all it takes is a custom memory controller that is optimized for this type of extraction.