Delta has been insourcing work on smaller and midsize CO ramps for years now. Delta has a subsidiary called DGS, Delta Global Services, an underwing contract company and they are the largest contract provider to CO for underwing services. DGS also provides underwing and customer service services to other major and regional airlines.
Delta mainline is also insourcing above wing and maintenance operations for several airlines. Have not heard of CAL taking over for Delta underwing, but with the way the industry is today nothing surprises me anymore.