That is true. But there are also such things as holding another person at gunpoint and ordering them to do something. It doesn't make them the same person as you. Their preferences are different even if they seem to behave in your interest.

And in either case, you are technically not deciding the other person's behavior. You are merely realigning their incentives. They still choose for themselves what is the best response to their situation. There is no muscle now-you can flex to directly make tomorrow-you lift his finger, even if you can concoct some scheme to make it optimal for him tomorrow.

In any case, commitment devices don't threaten the underlying point because most of the time they aren't available or cost-effective, which means there will still be many instances of behavior that are best described by non-exponential discounting.