Correct me if I'm wrong, but I don't think actual 70kwh battery could be paired to a car by Tesla and display "P85" without some non-standard changes on the MCU side. I'd expect the pack label is incorrect, but would be worth a CAN dump to verify.
I've tried this as a potential cost-saving solution for P85 owners who just want to be able to get driving again, and this works fine for non-performance cars. But the SPORT type drive unit cars (RWD or AWD) won't run on 14-module packs.... with the notable exception of the newer 14-module 90 packs where Tesla cheats and bumps max output current in the drive unit firmware to account for the lost power from the lower voltage of these packs (which I think is shady, since it increases wear on electronics 12%, but it's not the biggest issue I've ever seen with Tesla).