The ticket doesn’t need to be a DL-marketed ticket. A non-DL ticket for a same-day DL-marketed-VS-operated flight from airports with DL Sky Clubs also works to access the DL lounges at such airports with DL-marketed-but-non-DL-operated flights as an Amex Platinum/Centurion card presenter.

The important thing is that the non-DL ticket must have the flight and boarding pass showing the flight to be a DL-coded or DL-operated flight regardless of ticket stock.

Delta's rules allow entry with a "Delta marketed" flight. The fact that the non-DL-operated flight "also" operates as a Delta codeshare is not relevant, unless you purchased the Delta-coded flight.