smooth, but not flat
Therefore, not water.
These guys don't know what they're saying, imho.
There were plenty of similar patches that the rover drove through ands left wheel marks in the dust
There were plenty of similar patches that had various types of spectroscopic and mineral scans done to examine the nature of the dust
There were plenty of patches that had IR temperature scans done to show their temperature at various points in the day.
The presence of liquid water would have screamed out in these various scans.
Sorry...