Yes they can. However, they are technically renting inventory that they own, so it doesn't explain no availability. IMO, no availability comes from lack of planning by the owner. HGVC is a pretty good system for availability and if you are planning 9 months in advance then you should be able to get just about any resort. Certain times of the year and certain resorts are harder to get.
If you want Ocean Oak in the summer, it is tough. Except for the odd cancellation, forget about any reservations after the 9 month mark. Locations where HGVC has a lot of resorts, like NYC, Hawaii, Las Vegas and Orlando are pretty easy to get reservations.