Small print in Sony's North American version of its "The Best Place to Play" promotional video, spotted by Shacknews, mentions the six-month exclusivity period.
Ubisoft and Sony are yet to comment.
Assassin's Creed IV: Black Flag features an hour of additional content on PlayStation 3, focusing on Assassin's Creed III: Liberation protagonist Aveline de Grandpré. The three new missions take place after the conclusion of Liberation, and are independent of Black Flag's main campaign.
Ubisoft is yet to announce the specifics of what extras it will be including with Watch Dogs.
The current-gen versions of Assassin's Creed IV: Black Flag will launch on October 29, with Xbox One, PlayStation 4, and PC versions following in November.