There are a couple of limitations. One is that Golds only get the guarantee for North American hotels not worldwide. (But you are a Plat so you don't have to worry about that limitation.) I'm also not sure that all International hotels have to abide by the guarantee. I think there are some exclusions for them.

Another limitation is that the hotel must have king beds to offer. I do believe that at least some if not most RIs don't seem to have king beds. So if that is the case, you should get their next largest bedsize (Queen for RIs I do believe.) There may also be some international hotels that may not have king beds.

But otherwise, my understanding is that you should be guaranteed a King bed. It might be worth a call to Reservations and ask them. Maybe there is some other exclusion that may allow them to not have to abide by the guarantee.