"It naturally follows that, to minimise the time difference between sounds arriving from both units, the tweeter should be located inside the lf unit's voice coil. The drivers would then be said to be not just "co-axial", but "co-incident". The practical constraints of the tweeter magnet made this very difficult. If the magnet size of the hf unit could be reduced, then a truly co-incident driver could be realised." quote from KEF Website.
The primary design philosophy is to create a singe point source for the majority of the frequency range to avoid phase and time alignment issues found in standard direct radiating speaker designs with multiple drivers.
Just thought I would clarify. Nice article on a very good sounding speaker system. Definately a speaker system that should be auditioned if you are in the market. This is one of their more affordable systems.