After a quick look I feel that they did not actually test hammocks, but a hammock like bed. The fact that sleep onset was more rapid does not in itself argue for better sleep, and the only thing they studied was short naps, not a full nights sleep.
One thing I dislike about my Hennesy is that is makes it hard to move around in your sleep (just like the waterbeds are reputed to do). The "rocking bed" in the experient did not have this effect, and the short duration masked it anyway.