I ve got a Western Hauler bed, and while I've never been overly thrilled by how stiff it is ( of course, an air seat cured that
... keep in mind, I have no discs left in my back so I REALLY feel any little thing ) but I'd be hesitant to load some of the things I've hauled with a loose mounting setup like that.
Even the buses that use springs at the occasional corner , still have rigid ubolt mounting along the frame rails.
My bed is rigid mounted much in the way described in that manual you found , It is raised up off the frame rails and only contacting at the mount points and doesnt have paralel running rails running on top which I could surely see as an over stiffening point begging for problems, plus on the western hauler, teh frame of the gooseneck hitch goes across the frame mounts and IS part of the bed where they are all welded together, where the entire bed is 3/16 diamond plate so the entire bed becomes part of the hitch . It's built to haul ANYTHING, which I really like.
Sometimes when you have something like a flatbed, you just have to learn to slow down when you get off of a road, or at least get rid of the overly abusive gabrial or monroe or rancho shocks and put something on there like some bilsteins that will let you drive on rough surfaces without tearing your truck (and your body ) apart and still be able to control the load going down the road