The chronological placement of Mission to Zila is not entirely clear; Alex Winger is identified as being twenty years old, and Grand Admiral Thrawn is said to have recently taken command of the Imperial military, both of which would suggest that the story takes place in 8 ABY; but Coruscant is said to be still in Imperial hands, which would impose a date before 6 ABY (42:2:17) as stated in The Essential Atlas. The reason for this is that Mission to Zila was written before X-Wing: Wedge's Gamble, the source which fixed the liberation of Coruscant at that earlier date.
Perhaps the simplest retcon is to suggest that the Imperials on Garos IV remained unaware of the loss of Coruscant, but that would not explain why the planet's well-organized Rebel underground would be ignorant of such a major event more than two years; an alternative retcon would involve supposing that Thrawn had actually returned from the Unknown Regions much earlier than is generally supposed, but this would require some other means for explaining Alex's age.