I had a front desk agent denying that late checkout existed last year. They dragged out the manager on duty who also said they weren't required to provide late check-out to anyone. I kindly left them with this URL, which I keep bookmarked on my phone:
https://www.marriott.com/loyalty/terms/default.mi and left for dinner. Upon returning, the manager practically fell over themselves apologizing and said the info they were given in their Marriott training was incorrect. (was it?) In the end it worked out. The hotel had been open for less than a month so I cut them a bit of slack and didn't take it further up the food chain since they did make it right in the end.
Per the actual Terms & Conditions (emphasis mine):
v. 4 p.m. Late Checkout. Platinum Elite Members and above may check out as late as 4 p.m. local time of the Participating Property. Members can request late checkout when making a reservation through central reservations, at check-in, via the mobile app (where available) or at any time during their stay. This benefit is guaranteed at all Participating Properties, except at resort and convention hotels and Design Hotels, where it is based upon availability. Marriott Vacation Club, Marriott Grand Residence Club, Sheraton Vacation Club, Westin Vacation Club, The Phoenician Residences, a Luxury Collection Residence Club, Scottsdale, and Ritz-Carlton Reserve are excluded from this benefit.