Swedish studio DICE created the Battlefield series and has generally been the main developer behind the series.
Further on, when explaining the role overview, it’s stated more directly that they would “manage the design team and design vision of a new Battlefield campaign.”
The first three Halo games, made by Bungie, are three of the most acclaimed first-person shooters of all time.
Though Lehto is specifically Game Director, not Art Director, his presence could do wonders for Battlefield’s visuals, and the quality of their future campaigns in general.