Answer:
It's a vehement expression of derision, disbelief, skepticism, etc. ("Savages," she snorted). Or it could simply be a writer's substitute for "said," "stuttered," "shouted," "sneered," etc. It's seldom an actual snort, like a horse or someone inhaling a substance. One entertainer has practically built a career on snorting: Lily Tomlin, with her character Ernestine on Laugh-In. Look it up on YouTube. The "snort" often expresses disdain, so it doesn't need a modifier. A writer may add "derisive" because he or she is trying to be comical. It's probably overkill to say "he snorted in disgust" since dialogue and the situation should clearly convey the disgust.