While it is true that will always result in a winning line, it’s not true that it is the only way to force a win. Half of their moves will allow you to play adjacent to you starting corner towards an open corner and still force a win, as long as their first play isn’t the opposite corner or any of its 3 adjacent spaces. In fact, if they start in one of the adjacent sides or non-opposite corners, you have 3 winning moves. If they start on a side, you can take either the open, non-opposite corner, the side leading to that corner, or the middle. If they start in a non-opposite corner, you can take the first two moves above, or the opposite corner.
While it is true that will always result in a winning line, it’s not true that it is the only way to force a win. Half of their moves will allow you to play adjacent to you starting corner towards an open corner and still force a win, as long as their first play isn’t the opposite corner or any of its 3 adjacent spaces. In fact, if they start in one of the adjacent sides or non-opposite corners, you have 3 winning moves. If they start on a side, you can take either the open, non-opposite corner, the side leading to that corner, or the middle. If they start in a non-opposite corner, you can take the first two moves above, or the opposite corner.