So, if i'm reading this right - There's a guaranteed room type, and then the Ultimate Reservation Guarantee.
From Marriott.com:

Ultimate Reservation Guarantee

If for some reason we’re unable to honor your reservation, we’ll pay for your accommodations that night at a nearby hotel and compensate you for the inconvenience. To be eligible, you must provide your member number when making a reservation. Compensation varies by hotel brand.
So, they cannot honor my reservation, as the room is not available.

From Marriott.com:

Guaranteed Room Type

When making a reservation, be sure to note your Member number and room/bed preferences. We’ll always honor your bed type request at our locations throughout the U.S. and Canada. Everywhere else, smoking/non-smoking preferences are honored based on availability; exact bed sizes may vary, and only Titanium Elite members will have guaranteed priority for their requested type of bed and room. If guarantee is not met, guest compensation applies.
So, if I'm reading what ntamayo said, I could book a Presidential 1K Suite somewhere, but if it is in use and they put me in a Standard 1K room, then no compensation is due?

Surely, this cannot be.

This was not an upgrade, but an actual paid stay. I don't count on upgrades if I want a nice room.