Interesting topic. I've not heard that, or at least don't remember hearing it.
Holding that it is true, could it be that there is only a single variable in that theory and it is the dead space below the false bottom? If so, it is a function of the volume of water and grain being the same, therefore mash is effectively thicker when the false bottom creates more dead space.
In other words, you have 20 lbs of grain and 8 gallons of water. If your dead space is 1 gallon the mash is effectively thinner than if you had 4 gallons of dead space. Thinner mashes are supposed to be more efficient.
I'm not sure I could explain the theory in another way, and its just a wild-assed guess anyway.
-This is EXACTLY my theory too. Fairly early in the mash the enzymes migrate from the grain kernel's aleurone layer into the mash liquid and I'm wondering if they then just sit in the space under the false bottom unable to convert any starches because they're not in physical contact with the grain at that point.
Apparently thinner AND thicker mashes are less efficient beyond a certain point; it's that nice middle road that we want.
-I wonder if this changes when the wort is kept in constant motion ala a HERMS or RIMS mash.
I own a few brewing text books and I haven't come across a justification for WHY mashes with a greater space under the false bottom are supposedly less efficient. -I also haven't seen any brewery engineering recommendations for the percentage of the mash that SHOULD reside below the false bottom in an ideal situation; this is really what I'm looking for at the end of the day.