Check the discussion on the limitations of the TES instrument. In short in actual *field* experiments, analogous instruments were not able to detect quite large deposits of evaporites, even ones that were 100% carbonate.
To really seal their case, Christensen and Bandfield need to take duplicates of their TES instrument in the *field*, not just in the lab, and see how accurately it can detect *real world* samples.
My guess is they'll find the same thing other scientists have found with similar instruments: simple surface roughness and light dust coverings can obscure the signature of evaporites.

Bob Clark