John Rehling another participant on that forum argued there are some features shown by the Mars gullies that would not be shown by dry dust flows:

Water May Not Have Formed Mars' Recent Gullies.
JRehling Mar 16 2006, 09:33 PM
"It seems highly reasonable that dry mass collapses would not show
the branching of channel piracy (which, if the flow were spreading
upwards, would not branch in that way for any obvious reason, and would
lead, instead, to a broader upslope collapse -- more alcove). Still, it
would be interesting to see simulations using martian parameters
instead of relying upon earth-condition analogues."
...
"Just to be clear, I am not referring soley to the
alcove-channel-apron morphology, which, especially for a wide array
of equatorward-facing examples, are attributed to dry mass movements.
However, with respect to the Malin and Edgett seepage sites, which
are observed finer scales, "the observed morpholog[ies]" (e.g.,
sinuosity, anastomosis, incision, streamlining, presence of levees,
channel/stream piracy, etc.) are indeed "incompatible with dry mass
movement."
http://www.unmannedspaceflight.com/index.php?s=&showtopic=2428&view=findpost&p=45991

Bob Clark