Apparently they screw onto normal freewheel threads. However, a chainwhip wouldn't work because the gears do have a bit of freewheeling ability. It takes a bit of force, but the cogs can be spun backwards by hand.

We had an FFS equipped wheel come into the co-op with broken drive side spokes. At the time we couldn't figure out how to get the gear cluster off, but I did some research afterwards: The axle needs to be removed, and then a freewheel socket will fit in and unscrew the cluster.