Why the first heavy-gun hit can decide a duel

Does whoever scores the first hit usually win a battleship duel?

Often yes: an early heavy hit can disable exposed fire-control equipment and make it much harder for the opponent to recover accurate fire.

The most vulnerable items on a capital ship are often the exposed rangefinders, directors, and control positions needed to keep firing accurately. A first hit may therefore degrade the opponent's ability to reply before later salvos arrive. The point applies across the capital-ship comparisons raised in the wider exchange rather than depending on one particular pairing. Armour still matters, but it may not recover the fire-control capability lost before the next salvos arrive.