If you scroll down on that page and read the asterisked points, it clarifies the benefit of late checkout as for some statuses, late checkout is indeed based on availability. For Plat+, the asterisks clarify that it is based on availability at resorts and convention hotels. Adding Guaranteed to that line would also be incorrect as it is not guaranteed for Silvers. Instead, that line should probably just be removed, but honestly, when in doubt, the terms and conditions should be your source of truth (and they specify things correctly).

What property was it? Perhaps, it is designated as a convention hotel.