Sharing a recent experience booking a non-refundable THC rate through Amex portal (and using the $300 credit):

• Stay was not recorded in the Hyatt app at first but a quick chat with the Concierge solved it
• Was given full THC benefits - including early 12pm check in, 4pm late check out and $125 dining credits (usable even for room service)
• Was only upgraded to a "deluxe room" which had obstructed views of the Singapore Flyer (did not mind since I was there for work)
• Elite nights and hotel spend (including the THC room rate) were recorded in my account 24 hours after check out