Had a similar experience. Tried to enter with Amex, but under interrogation, they found out I was flying KL business, after which they were intent on using my boarding pass (and only my boarding pass) as a credential for entry. I gather reimbursement for Amex/CapOne cardholders is materially lower than Virgin/SkyTeam/El Al business passengers, so staff is encouraged to exhuast non-CC entry credentials before swiping a card.

Agree that the club is worth the effort to attempt an entry. Food much better than Centurion, although the atmosphere is a bit characterless IMHO. As I related to a client recently after using Amex for access prior to DL domestic flight: "I appreciate the access, but it feels much more like a contract lounge vs Upper Class prelude..."