It has nothing to do with guilds, he was simply not an editor. Every director is involved with the edit, making decisions about what to cut and keep and even sometimes when and where to actual cut between shots. Some are more hands off and some are more hands on. Since Lucas comes from an editing background he was more heavily involved in the edit since he was shooting for the editing room to begin with; I don't think he should be considered an "uncredited editor," because if thats the case then pretty much every director should be.