First, consider a simple one element lens, like a magnifying glass. The distance from the lens to the "sensor" is the focal length. A long focal length will be more distant from the sensor. (this is the "thin lens" formula).
However a such a simple lens has aberrations, optical faults (primarily, a single element cannot focus all colors into the same spot). It would be poor lens for photography. So additional corrective elements are added. This thickness results in two focus nodes, one node seeing the scene, and one "seeing" the sensor (and even more elements added to transfer the image between the nodes).
The nodes are typically inside the lens body, but math can place those two nodes anywhere. Telephoto means the scene node is in front of the lens, so that the lens is shorter than its focal length (to be compact, easy to carry and use). Telephoto does not mean distant, wide angle lenses also focus at infinity.
Retrofocus lenses (more extreme wide angle) puts the sensor node well behind the lens, which allows room for the mirror of a SLR to rise behind the lens.
Mirrorless cameras have no need for that extra spacing, so the advantage is simply a smaller camera body. It is NOT an optical advantage.