The difference is they traded the speed for range. It's 1000-fold slower than zigbee (0.1 kb/s) but with a bigger range. They say they can cover whole France or California (including unhabited areas) with only 1000 antennas, thus cheaply creating a global network. With zigbee, you can really do only a local network and still need a local relay to internet.
So, Zigbee allows you to make a fast, fee-less, local sensor network. While this is a global network with yearly fee designed for low-bandwidth sensors.