Ya can see it in this shot of him, so however he got it, it must have either come down vertically(most likely) or it went up vertically(not too likely). Now, this scar is on the very border of his left eye like with Scar so, how is his eye itself un-injured? It seems like in a normal situation his eye would have also been hurt, and he would most-likely have lost eye-sight in that eye.
What it looks like is somehow his eye escaped the hit despite how parts of it are directly on the top and bottom edges of his eye. Unless his eye was able to be covered by something, or his eye just "went out of phase" I don't see how his eye is okay.
