All other factors being equal, a more closed racket face does increase topspin. It allows for asymmetrical force on the top half of the ball, depending on the nature of the incoming ball.
For ease of understanding, imagine a perfectly horizontal incoming ball, which has no spin on it. Now just block this ball with the racket face. If the racket face is perfectly vertical, then the ball will be struck in the middle, resulting in no spin and no downward or upward deviation from the horizontal plane (we're ignoring gravity and string deformation etc cos they don't change the underlying trends).
Now if you close the racket face slightly and block the same horizontal ball, it will result in the ball coming off the racket downwards and with topspin. Why? Because of the asymmetrical force aplied to it. The top part of the ball is blocked, while the bottom part keeps going, which results in the spin.
Conversely, if you open the racket face slightly to block the same ball, you will end up with an upward trajectory and backspin, because this time you're blocking the bottom part of the ball while the top part keeps going.
This is the reason why the more open the racket face, the more vertical swingpath (and faster swing speed) you need to achieve the same topspin