From the SRD:
"Weapon Proficiency: Elves receive the Martial Weapon Proficiency feats for the longsword, rapier, longbow (including composite longbow), and shortbow (including composite shortbow) as bonus feats."
Elves are skilled in both the traditional longbow, a self-bow and the composite bow. They use entirely different methods of construction and they shoot quite a bit different. A composite bow can in fact be 'tuned' during construction to have a number of different final characteristics. This difference, between a standard longbow and a composite longbow, would be no different than the difference between a longbow and an aquatic longbow. If an Elf can effectively ignore the difference between a self-bow and a composite bow, the difference between a longbow and an aquatic longbow would be trivial.
Shove that into your GM's pipe and have him smoke it.
This twit has annoyed me...