I have to say this may be an interesting strategy.
Perhaps DL feels that Southwest will so undermine US that US will greatly cut back its PHL fortress hub, or may not survive.

The full fare business traveller in PHL currently flies US to reach most cities. Southwest may be great, but it isn't a full network carrier, and there are an awful lot of places you can't get to on Southwest. Perhaps DL believes that they can lay the groundwork to pick up high value business pax through the Atlanta hub once US is mortally wounded.