I heard back from the seller. I had given him a link to the NCV calendar showing that 7/3-10/09 is a Platinum Plus week.
He apparently posted his available weeks as #23-51 based on telephone discussions with Owner Services. He also apparently was somehow able to reserve that Platinum Plus week, confirming to me that the check-in date is July 3. He gave me some details of his conversation with Owner Services when he made the reservation and his comments seem plausible, even if not logical under the reservation rules. He also confirmed that he has a Platinum week, not a Platinum Plus week.
He is very willing to give a buyer a copy of his reservation info as well as info that proves he owns deeded week #23 (a Platinum week), as stated in the ad.
Still seems strange.