Not GCMS
The study I referred to did not rely on GCMS but instead took biological cultures from the soils and multiplied the bacteria to a level of easy detection. This gets round any issues of minimum detection limits with GCMS technology, whether modern or ancient, Viking, or terrestrial.
Of course, you can always invoke a "different" organism that won't culture in warm wet environments, but until such a superbug is proven, the Atacama samples look pretty sterile.