As others have noted, this flight wasn't operated by Delta, even though the airplane and your ticket and the website where you bought it all say "Delta".

That might seem like a marketing distinction only, but no part of flight operations can be mixed, including crews. No "777 pilot" could ever have piloted that flight. And there weren't spare pilots in Atlanta (as crazy as that might initially sound when seemingly flying on Delta), because the airline that operated your flight doesn't have a crew base in Atlanta.

In fact, the majority of Delta-named flights in the United States, slightly over half, are marketed as Delta but operated by partner airlines under contract to Delta Air Lines.