Swissair (and the whole Qualiflyer Group) had and still have a close link with Delta. SR was not very pleased about Air France's links with Delta.
I have no concrete knowledge of Delta and/or Swissair breaking their close relationsship (they are still even shareholders across).
The Swissair-Group is an attractiv bride for all big Alliances (one underestimated force of Swissair is their 49% stake in belgian Sabena - and Sabena's strength in the african market and the african market is underrepresented in the existing alliances).