Correct, a steel starter strip will not stop water, but the proper angle of the bottom piece will stop any capillary action along the back side of the siding. I've never used the pre-finished product though. So that fin set the angle? And all the starter strip is going to do is help lock the fin in at the bottom so it can't move? If a rodent did get behind the bottom row of siding around the fin that is pushing up against the sheathing, where would it go? I wouldn't worry about it. My bottom row of LP is blind nailed at the top, sits over a 1 inch ripped piece of siding as starter strip which isn't totally continuous because no one will ever see it and I've never had a piece move, nor a rodent get behind it. A mouse is just going to find a door jamb to slip through before trying to get behind the siding. But I would not face nail it.