fuji used to make a p&s, the natura, with a 24mm f1.9 lens. That lens had a massive rear element, about the size of the whole frame, that went so far back into the camera that was actually touching the film. You can see it in this video at about 1:50
https://www.youtube.com/watch?v=9gDtaWfcESQ
I *suspect* that the x100 lens is modelled after this as it shares many specifications and design: about half the lens is inside the camera and it has a massive rear element (about the same size as the sensor) that must be extremely close to the sensor
IMO it's a miracle it even exists and works as well as it does