Update: the property proactively sent me a booking confirmation for the Marriott (stay is still several days out) so I asked if Ultimate Reservation Guarantee benefits would apply here. They said yes, and are providing me with the comped room, cash, and 140k points. I am rather shocked they are doing this so far in advance rather than at checkin, as one would have thought surely they'd have some no-shows and/or lower tier status people they could walk at no cost.

I can only surmise that either they have a large VIP block using nearly the full property past their cancel date, or perhaps that hotturnip is correct that they just want to avoid ugliness at checkin being a Ritz. Maybe both.

Either way, I am satisfied at the way they handled it. While obviously I'd prefer to stay at the Ritz all else equal, the property was upfront and professional about it, and are fully honoring the URG T&Cs rather than trying to weasel out under some kind of "we gave you advance notice" BS. And it is much preferable to have it sorted out in advance rather that showing up late at night and dealing with the hassle of a walk then.