The Diamond "forced" reservation guarantee only applies to a revenue room at the rack rate and even then has limitations (e.g. the hotel cannot be more than 110% sold or 102% sold for certain properties). I don't believe there is any guarantee that you can force a booking at a sold-out property on points.

Your best bet is, while signed in at your account, try to book the room - If you are able to book with points, the offer will be available, otherwise you will either be offered the rack rate (if less than 110%/102% booked) or told no rooms are available (if more than the threshold)