The original idea behind Spring Drive was to eliminate the Swiss lever in a mechanical watch. The continuous movement of the seconds hand is just a byproduct of choosing the most efficient power release method which is the inertial brake in a spring drive. The quartz crystal and other components merely regulate the inertial break for enhanced accuracy. A well adjusted spring drive is on par with a standard Swiss lever even without the quartz crystal.
In conclusion the crystal there is just for added performance. The watch itself combines best of both worlds and it is definitely a mechanical watch (that is quartz regulated).