I'd say as long as the arrow is moving it could seriously hurt or kill something. Try simply dropping it point first (with a sharp broadhead of course) from maybe 4 feet onto a piece of roast and see if it penetrates at all. Now toss it like a dart and see what it does. "reaching its arc", is a different thing. Gravity is pulling the arrow to the earth at all times.
If you truly launch the arrow (or bullet) parallel to the earth, it will continue to "drop" the whole way until it hits the ground, no arc, just drop. Meanwhile, the arrow is still going fast. . fast enough to hurt or kill. Even at 100 yards.
ChuckC