That type of card is addressed on the website, but not in a way that is helpful:
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I would probably have paid the $50 fee and asked the issuer to reimburse me. At this point I would ask the issuer if you are eligible for free admission. If you are, ask them what you should do if turned away.
The Centurion Lounge website has been edited several times since it was first published, and I think that previous versions were more useful on this point, giving some examples of excluded cards such as the Delta Skymiles Platinum Credit Card. The attendant is correct that not every platinum-colored card, or card with the word platinum in the name, is eligible, but that doesn't tell us if yours is. If it were as simple as a charge versus credit card distinction I think they would state that. Probably has more to do with annual fee amount.
Thank you for your reply. They did question me about the card's annual fee amount, in my case roughly $550/year (USD converted). They let me in for free on this visit but not likely on future visits. FWIW, I don't feel it would have been worth the $50 given how busy it was, there were hardly any seats available...