There is language in the Rewards Terms & Conditions referring to an "eligible rate," so some rates can be excluded from earning points. ("A qualifying stay is any night stayed in the current calendar year at a participating hotel at an eligible rate including Marriott Reward Redemption stays and paid for by the member.")
Also, in such cases, a Platinum member would not receive points as an Arrival Gift -- but would still receive other benefits, including the F&B option at Autograph Collection and other brands that offer this. ("Members staying on non-eligible earning rates may choose the F&B option only, excluding The Ritz-Carlton, EDITION and Marriott Vacation Club which do not offer the F&B option.")
I don't see anything in the Rewards Terms & Conditions that allows a hotel to exclude a Gold Elite or Platinum Elite member from being eligible for other published elite benefits, regardless of rate. Perhaps I'm missing something.
That being said, the biggest problem here is that the hotel on Marriott.com apparently failed make it clear to the OP at the time of booking that there would be no Elite benefits when the rate options were displayed. The OP is Platinum Elite or Platinum Premier Elite and knows what to expect at an Autograph Collection property (and other Marriott full-service brands). When the rate options appeared, Marriott.com and the hotel had an obligation to make it abundantly clear that the rate would not earn points or provide Elite benefits. Such a material decision factor is not something that should have been hidden in fine print in the "rate details" or other non-obvious place. (How often do we carefully analyze the "rate details" link when we make a routine reservation online using rates for which we are eligible?)
Shame on THE PLAZA Seoul, Autograph Collection.