I don't see anything unusual about making a game on the same engine. If you poke around other titles, including many that are seemingly unrelated, you might be surprised. Likewise, it couldn't be an add-on because a lot of the engine has been updated so it wouldn't run on the same exe (as an add-on normally would).
As for last year... imagine if the devs had even less time to work on it. Oooh boy
If it weren't a separate game, I doubt Ubi would be willing to finance its development at all. It's got far more new content than many separate games out there, so I don't see a problem here either.