When V3 supercharging deployed, even if the current S/X battery packs can't take full advantage of the faster charging, if the V3 chargers use the same strategy of sharing two charging ports on a single charger, the older packs should benefit because they may be able to charge at the full rate, even if another vehicle was already using the other shared charger.
The 3 packs have a different design. Tesla has been talking about V3 supercharging since before the 100 packs came out. While it's possible they incorporated the pack changes needed to support V3 supercharging into the S/X 100 packs, it's more likely they did that with the 2170 packs, which take up less volume for the batteries, providing more space for increased thermal control and handling the higher charging rate.
Since the current S/X 100 packs use the older batteries, that take up more space, it seems less likely Tesla would have been able to make enough changes in the design to handle the faster charging rate and increased heat caused by that.