The ship is still cruising at 22 knots and could use its full speed to disrupt enemy aim. Instead, it has closed far enough for two-inch weapons to affect opposing battleships. Those guns were included for historical plausibility, with no expectation that they would matter in a capital-ship engagement. Their participation is a warning that the ship has compressed the range so far that its long-range advantage is being squandered.
Notable Point · 36:39-37:35
When two-inch guns join the battleship fight
Light guns entering the action warn that the modern ship has closed far nearer than its long-range advantage requires.