These batteries are in a circuit which has a physical power switch separating it from the device, meaning there should be zero parasitic drain while off.

Thus I conclude your fun fact to be apocryphal.

Then why have I had Duracells leak in an unopened package before the expiration date?

That doesn’t make sense. Every alkaline cell suffers from this due to the formation of hydrogen gas inside the cell over its lifespan, as well as corrison caused by the chemistry of the cells themselves. It’s not specifically driven by the charge of the cell; although that is certainly a contributing factor and poorly designed circuits will exhibit more wear on the cell causing more rapid corrison & gas formation.