If the mower shroud is not close to the ground the ground underneath the
flail mower does not create the restriction required to create the pressure
gradient needed in front of the flail mower rotor to create the lift needed
to carry the clippings up and over the flail mower rotor and back to the ground.
Always think of it this way; if your upright vacuum cleaner with the carpet beater brushes
is held above the carpet while trying to clean it the vacuum cleaner will be sucking only
air and very little dirt except for the occasional dust bunny that wants to sacrifice its self.
I have no idea where you're getting this theory, but I'd be willing to bet a scientific analysis would disprove it....and the vacuum analogy doesn't work either.
Even with the mower shroud touching the ground there are far too many areas that are wide open (the entire front of the mower) for there to be any significant suction caused by the spinning rotor and blades. There is air movement, but it's not suction caused by a seal with the ground. Further, as you describe, the clippings are already cut if they go up and over the rotor...suction won't change anything...they're already cut.
A vacuum uses a suction motor to suck up what the rotor brushes lift off the ground....a flail doesn't work that way.
A flail needs to be near the ground for the best cut because of mechanical factors. The front of the flail (usually the metal or rubber guard) hits the grass/brush and pushes it forward (assuming it's long enough) and the blades then cut it. If the flail is up high (8" example) the grass and brush will be more flexible and it's going to move around before it gets cut. It's possible that in some situations it will bend enough that the blades miss some pieces entirely.
In short, if the mower is down low, the brush gets pinned in place better before it's cut which is going to give the most consistent cut. I've been watching my flail as I cut a lot...since it's new, and if you watch what happens to the brush as it goes under the mower, the tops are moving all over the place as they get run over. The bottom can't move because it's in the ground. Cutting closer to the fixed part is when you get a nice cut, but it's simply a matter of mechanics, not suction.
Regardless of all of that, if you drive through a field that has 36" tall weeds, and you set the mower at 8" above the ground, it's still going to cut the weeds...just not as smooth of a finish.