Earlier quoted context omitted.
Well, linguistically speaking, adding the suffix "-less" to a word implies "without", in English. You can be penniless, having not one penny. You can be homeless- not just fewer homes but by golly none at all. You could be endlessly fearless and remain motionless when facing things that aren't harmless. Doubtlessly, I've been tasteless in this reply.
If English followed stricter rules "stainless" would mean "without stain", and to convey "unable to be stained" we would always use "unstainable" instead, a real but rarely used word.
It does mean that (and/or "unable to be stained").