Some research on Molybdenum Disulfide and I find that it is both a dry lubricant and corrosion inhibitor. Interesting because what you've seem to find is corrosion in stainless steel where moly had been used.
However, this article
http://www.cor-pro.com/corrosion-protection-services/coatings/molybdenum-disulfide/ seems to suggest that those properties are present when moly is mixed with certain resins, binder and water soluble sulfides to improve it's corrosion inhibitor properties.
And this brings to mind the question of whether all moly is the same when it comes to bullet lubes. Are we using the best moly? Or is the less expensive moly also less able to inhibit corrosion?
Also brought to mind is the question of whether the barrel you inspected as was ever shot with copper jacketed bullets and what, if any, possible interaction copper and moly can have on a SS barrel, especially when combined with humidity or cleaning chemicals.