You are all partially correct. In Atlanta the "wheelchair jockeys" are indeed Delta employees. Technically they are employed by
Delta Global Services which is a wholly owned subsidiary of Delta Airlines. DAL Global Services also employs the SkyCaps, aircraft cleaners, and baggage handlers. So in a way they are contract employees but they aren't at the same time! DAL Global also provides contract services to other airlines mostly at Delta hubs and some outstations.