Note that we see a lot of complex polyaromatic hydrocarbons in interstellar dust across the galaxy. And we know these can sometimes have quite long alkane chain

Note that we see a lot of complex polyaromatic hydrocarbons in interstellar dust across the galaxy. And we know these can sometimes have quite long alkane chains attached or "inside". I don't think we can rule out stuff like that decomposing at those GC/MS conditions and giving the same signal. And that's even before the leap to fatty acids which may or may not have a biological origin.