The design work was complete long before anyone working on this project was hired by Ubisoft, and proven in the form of a game that shipped several console generations ago. Ubisoft presumably still has all of the original art assets for reference.
All that had to be done was to study the original game code, port it for modern systems, and then polish up the visuals some. Not a trivial amount of work by any means, but much, much easier than starting from scratch and making a game from nothing.
This should've been a layup for any competent studio given SEVEN YEARS(!!!) to work on the project.
That it wasn't, is undeniable evidence of a AAA game development competency crisis.