That's for lower levels - for Platinum it is Guaranteed except at resort and convention hotels where availability kicks in.
Had a property deny this. A quick call to customer service and the hotel resolved their "misunderstanding"
From T&C
" 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 HotelsTM, where it is based upon availability. Marriott Vacation Club, Marriott Grand Residence Club, and Vistana properties are excluded from this benefit.