Don't retrofocal designs increase the back focus distance? How does reducing the back focus distance eliminate the need to increase the back focus distance? If the goal is a shorter back focus distance, then you are talking telephoto, not retrofocal, right? Maybe not.
Broadly speaking, that is correct. However, the need for a retro-focal design is linked to the focal length of the lens and the desired image circle it must project.
I'll explain this with an example: a 50mm lens is a 50mm lens because that is the distance from the front element to the focal plane (e.g. the sensor or the film). Now, in the real world this 50mm generally includes a fair bit of the space inside the camera ... where the mirror swings in a Canon DSLR. Not enough space, so we must move the lens forward, i.e. increase the back focus distance. The answer is the retro-focal design.
Now, as the EF-S mount has a shorter back focus distance (smaller mirror), one could design some lenses without resorting to a retro-focal design. (This is also why mirrorless is so appealing in terms of lens designs.) Or use less of a retro-focal design. Less elements within a lens is always better ... up to a point, anyway.
Another factor is the effect that as the image circle becomes smaller, as with an APS-C sensor, the minimum focal length where one must start using a retro-focal design also decreases.
Plus, the EF-S mount allows for a fair amount of cheating in terms of the size of the optics, but that's another story.