I was curious about that statement also.
What I do know is that all F25s, including those built before Auto Stop/Start was added, use an Absorbed Glass Mat (AGM) battery technology, rather than the more familiar Flooded Lead Acid technology. AGM batteries are available after-market, and I'm sure it's possible to find one with the right specifications. However, there's more to replacing the battery in an X3 than just replacing the battery. There's is also software that needs to be set up, including registering the new battery and resetting battery information in the X3, work that is best done by a BMW dealer.