They're an incredibly innovative company. As an innovative company they try out lots of different ideas. Most of those ideas are going to be relegated to the dustbin. But the ones that work can turn into billion dollar businesses.
Using car boots for delivery might not work with a single manufacturer, but it might work for all manufacturers. It might also work more effectively with driverless cars. Either way it's an experiment to push the boundaries of what can make their supply chain more efficient.