That is a good analysis, but also consider that the instrument in question bakes the samples at 1800F before measuring. Molecules like methane are broken up at that temperature, and I doubt anything like DNA would survive intact. On earth, that temperature is used for dissociation on methane as a feedstock chemical for manufacturing other chemicals, like ammonia. So, they are only going to detect some base molecules and atomic signatures, but those signatures may be in the proportions that point to things like methane or other organics.
THAT's very hot! Yes, many molecules will break up at that temperature.
We're talking hydrocarbons, here, I think. I'd be willing to bet on it (but only a cup of coffee!:-)). Fats are hydrocarbons. Methane is a hydrocarbon. You know what people will think of when they hear that word?... There's oil on Mars!!!