I do see a No 1 lounge listed for Skyteam Elite Plus, but it's for Gatwick, not Heathrow. Maybe it's incorrect and they mean Heathrow. Generally, DL Skyteam Elite Plus flying DL don't get access to third party lounges, so I'm guessing there may be some special arrangement in place. Also, I do not see any carve out allowing US GM's to access VS Clubhouse's. DL website still says Plat and Diamond (or D1 pax) only on TATL flights to/from the UK only.
Well I don't know what part of the DL website says that; here's the part that I referred to:
"Clubhouse lounge access: Virgin Atlantic Clubhouse lounge access is available at select locations to Delta One™ and Upper Class passengers and Diamond, Platinum and Gold Medallion members with a same-day nonstop transatlantic flight between North America and the U.K. that is operated by Delta or Virgin Atlantic. Diamond and Platinum Medallion members may access Virgin Atlantic Clubhouses in London-Heathrow, London-Gatwick, New York-JFK, Newark, Washington-Dulles, Boston, Los Angeles and San Francisco Airports. Gold Medallion members may only access Virgin Atlantic Clubhouses within the U.S. (New York-JFK, Newark, Washington-Dulles, Boston, Los Angeles and San Francisco Airports). Diamond, Platinum and Gold Medallion members are allowed to bring one guest. While access to the Virgin Atlantic Clubhouse in the U.K. (London-Heathrow and London-Gatwick) is not available to Gold Medallion members, these members may enjoy the No. 1. Traveller Lounge at LHR’s Terminal 3. Please travel with and be prepared to show your Medallion card to the Virgin Atlantic representative to receive these benefits."