So I called Marriott Rewards and they said that the hotel was incorrect and that I do definitely get elite benefits.

I then went down to reception and spoke to a different agent who said she didn't know why I had been told by the person checking me in that but that I do definitely get the benefits. She did not seem particularly apologetic about the situation. Anyway their lounge is now closed but I get breakfast in the morning. This hotel (Marriott Denver - Gateway Park) has not impressed me.