If you check the Apple support document a bit closer, you will see that Time Machine is supported on the new 802.11ac model of the AirPort Extreme.
Time Machine works with:
External USB hard drive connected to AirPort Time Capsule (any model) or AirPort Extreme (802.11ac model only)
That would mean that Time Machine is not (and has not ever been) supported on other models of the AirPort Extreme.
However, what the document does not tell you, is that the USB port on the new AirPort Extreme (and Time Capsule) is only USB 2.0....and it runs at about half the speed of a USB 2.0 port on a computer. So, if you connect a drive to the USB port on the new AirPort Extreme, backups are going to take about twice as long as they would compared to a USB drive that is connected directly to a Mac.