"You know him better. It stands to reason that you would seek him over myself." It wasn't meant as a means to scold him. Hanzo knows why Benedict would be the first person to be asked - he was the one who wanted to return home. They were there as escorts; in Hanzo's case he knew very little about the nature of the mission. He only wanted to see Tevinter again one more time.

It's obvious that the other man isn't fond of the answer, but Hanzo isn't going to settle down and tell him the truth. While he hasn't outright lied he has kept some things back, to protect himself and to protect Benedict. If the truth comes out before Hanzo has done what he can to fix it... It will not end well for either of them.

"If I learn anything relevant I will inform you." It's not the same kind of promise he had made to Benedict, but it's something. He can control the information, then, rather than worry about it being taken out of his hands. "And I will make sure that Benedict does the same."