The problem is we don't know how many layers there are of these monolayers. The authors say "multiple" but don't quantify this: hundreds, thousands, millions? We don't know. It is possible for example that every single micron-sized particle is covered by it's own monolayer of liquid water. This clearly would result in quite a bit of water. This is something that should and can be simulated in the laboratory under Martian conditions.
The phenomenon of "surface melting" is an manifestation of such thin liquid films. Most importantly this can occur on the ice between the individual grains in soil. This is the cause of such macroscopic phenomena as frost heave that results in ground being raised several feet in ice-filled ground.
Some references on surface melting:

FAQ - Melting Below Zero
"After a fall freeze, farmers in rock-ribbed regions such as New Hampshire may awaken to an upheaval of their previously cleared fields: stones stand on pedestals of ice needles, and soil bulges up around larger rocks. This occurrence (called frost heave) ranges from an agricultural annoyance to an industrial nightmare. Despite its dramatic effects, frost heave owes its strength to microscopic liquid films on the surface of ice.
"Frost heave begins when chilly air cools the soil and freezes some of the water near the top of the ground, but the real damage is wrought after this initial freeze. Molecular forces and impurities on the ice surfaces can prevent the moisture from freezing solid until the temperature drops several tens of degrees below zero Celsius. Until then, a microscopic film of water coats the ice crystals, which grow between the tiny fragments of rock and clay that make up the soil."
http://www.bushwalking.org.au/FAQ/FAQ_MeltBelowZero.htm

CAPILLARY EFFECTS ON THE STABILITY OF ICE IN MARTIAN CRUST, Paul C. Hess1 and John
Longhi2, 1Department of Geological Sciences, Brown University, Providence, RI 02912, 2 Lamont Earth Observatory, Palisades, NY 10965
"On Earth liquid water exists in porous soils to
temperatures as low as -40ºC below the bulk melting
temperature (7). The stability of liquid water at these
low temperatures is often attributed to the lowering
of the bulk melting temperature of ice by dissolved
impurities in H2O. Indeed, large concentrations of
dissolved salts are fully capable of explaining most or
all of these temperature depressions (3). But given
grain-boundary energy arguments, it is also possible
to show that liquid water can remain in equilibrium
with ice and/or H2O vapor even in the total absence
of such impurities. Liquid water, for example,
remains unfrozen in porous media such as graphite or
quartz powder to temperatures of –20C or more (5,6)."
http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1702.pdf

Ice surfaces: macroscopic effects of microscopic structure
By J. S. Wettlaufer
http://www.geology.yale.edu/~jw378/articles/JSWMillenium99.pdf
(a detailed review article.)

Bob Clark