The Type II stabilzer first used AEH, a type of sugar (a hepitatol), but this was found to be too variable, so it was changed to sorbitol. The sorbitol and the AEH increased the glass transition temperature of the gelatin and thereby provided a rather impermeable oxygen barrier. It made the prints stickier in humid environments too, and so it was recommended only for enlargements that were to be mounted not prints being stacked.
I'm not sure about that explanation. Glass transition temperature of gelatin is only lowered by adding small amount of sorbitol. Plus, I did this experiment myself some time ago and srbitol-treated gelatin is permeable to oxygen and peroxide. If sorbitol treatment indeed prolonged the image longevity I suspect it did so via radical scavenging action on excited dye molecues, or perhaps by scavenging the offending molecule or its intermediate forms.