This was news to me. Just got a denial.

'In order for a Competing Rate to be valid at a SPG Partner Hotel, the rate booked on your reservation must be booked at a published room rate that is currently available online or on a mobile application for the referenced SPG Partner Hotel and the same dates of travel. The rate lowest available rate on our Starwood-branded website must be booked, regardless of the number of guests, the room type, the view, the room size or any included amenities. "

The design hotel lowered their rate the next day and I received a claim denial because of this. I called the SPG line and they said this is normal because Design Hotels have their own rules and I would have to re-book at the newest rate and re-submit a claim.