There is a breakfast guarantee for Gold/Plats worldwide, but I do believe it may be a bit different for different regions. And that may have confused the Marriott agent who replied to you.
The applicable T&Cs: " For JW Marriott®, Autograph Collection®, Renaissance® and Marriott® Hotels in the U.S. and Canada: any time the Lounge isn’t open, Platinum and Gold Elite members are entitled to a choice of continental breakfast for two in the restaurant or 750 points daily. In Europe, hotels without a lounge offer a choice of breakfast for two in the restaurant or 750 points daily. If Concierge/Executive/Club Lounge Access and complimentary continental breakfast is not offered, Elite Members will be compensated $100 USD for the inconvenience. Hotels outside the U.S. will pay the equivalent in local currency."
It looks like in NA Gold/Plats are only guaranteed a Continental breakfast, where in Europe you get a full breakfast. And I've never been to an Asia FS hotel where I didn't get a full breakfast.