His lore describes him as the leader of the kinkou order, yes. The task he fulfills is that he is protecting the balance between the spirit world and the human world.
His class in the game is warden, which is a tank who excels at protecting. So I think it kinda fits his lore. And it would be pretty boring if we make every ninja in the game a hard hitting fighter or assassin. Ninja have more traits than just this.
Ninja are also known for magical techniques, which he uses to generate shields, teleport to an ally and protecting everyone from auto attacks. Not every tank has to be a big meat shield soaking every damage, you can as well be a tank by dodging or blocking skills from the enemy, which fits the ninja theme and is represented in Shen.
So, I personally don't mind him being a tank. It makes sense the way I think about it.