The thing about Brian Horton's role as Game Director is that he was only in that role since January 2015. In other words, he was in charge of overall game direction for less than a year, and for the larger part of ROTR's development, he was still art director.
Daniel Chayer was the person who was the director for the larger part of ROTR's development, before he left in March 2015 to go to Eidos-Montreal to lead development on an unannounced project.
Brian Horton probably prefers to stay in the art side of the business, but took on temporary leadership in the final sprint of ROTR's development.