Even then, all of chemistry DFT is based on the idea that the electron density contains the physical observable information and you and I both know that the overall phase of the wave function isn't physical except through interference. There is plenty of useful qm without C already out there!
This is referring to the fact that overall phase is not real (no observable difference) but relative phase has. The word “except” is not downplaying its importance, but to emphasize the fact that overall phase isn’t physical.
