The site tested the device with its display off and a single notebook tethered wirelessly and downloading at a constant 50KB/s.
I set the iPad Air up as a personal hotspot, wirelessly tethering it to a 13-inch MacBook Pro with Retina Display. I started a constant 100KB/s transfer on the MacBook Pro (2x the transfer rate of my iPad 3 test) and with the iPad Air's display off I measured battery life. Last time I chose 50KB/s as it was the average transfer rate across our old WiFi web browsing battery life test, I doubled the workload to be more reflective of more strenuous demands. In reality I'd expect to see a burstier usage profile, but that's something for me to test down the road.
The iPad Air lasted 24.08 hours before dying and had transferred over 8GB. Notably, this is more than most people's monthly data plan.