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.
And your second move has to be another corner whose line hasn’t been blocked.
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.