"Uber had placed a long-term order" [...] "Uber is particularly interested in autonomous driving vehicles, the magazine reported, adding that such cars are expected to be available after 2020."

Perhaps the long-term order is a commitment from Uber to motivate Mercedes to invest more heavily in autonomous vehicles? If the long-term strategy of Uber is to have a fully autonomous fleet then a long-term commitment of $5bn with one manufacturer seems plausible.