Black Flag's open world worked because of the ship. However, the actual landmasses were not big at all.
This rumor is basically a recap of what leaked last year however that leak mentioned that the horse and some form of boat travel were included, which means that the traveling could be mitigated.
The desert is probably what will be the majority of the map size, however what populates it is what will determine whether or not the open world size is justified. While I know the instant assumption people will have is "oh they are just going to copy/paste the same missions, spread hundreds of collectibles, etc"
But, ubisoft said that the new Assassins Creed would mark a new format for the companies open world games, so all we can do is wait and see.