The addition focuses on Aveline, heroine of Assassin's Creed 3: Liberation, and is said to add an hour of play time to Assassin's Creed 4: Black Flag. While it was initially announced as exclusive to the PlayStation 3 and PS4, the aforementioned promotional video stated that the exclusivity agreement between Ubisoft and Sony was only valid for six months.
A Eurogamer report today stated Ubisoft had confirmed the DLC would never appear on systems outside of the Sony umbrella. We reached out to Ubisoft for comment, and the company explained that the DLC could potentially reach other platforms in the future, but that currently the publisher had no plans to do so.
"I can confirm that at the current stage the PS exclusive DLC isn't planned for Xbox 360/Xbox One," Ubisoft told Joystiq.
The key word here is "current." Ubisoft appears to be keeping its options open for the future, while also adhering to the spirit (if not the letter) of its agreement with Sony. This addition may appear on other consoles at some point, or it may not - either way these PR vagueries absolve Ubisoft of having to commit to anything.