Sion ult collides with friendly Azir ult (wall)

Azir Emperor's Divide: "The soldiers then remain as a wall for 3 seconds. Azir and his allies can pass freely through Emperor's Divide."

Sion Unstoppable Onslaught: "When Sion collides with an enemy champion or wall..."

Is Azir's ult considered a "wall" for friendly units? If so, why can friendlies pass through it? I can understand the wall, the wall, Trump wall, etc.