Because the flyback voltage at the primary side rises to -several hundred volts peak. Put an oscilloscope at the coil - connection where the points or ECU connects, and you will see this.
Theoretically true, but don’t the ECUs have snubbers that limit the primary voltage peak to a few hundred volts to prevent breakdown of the switching transistor? That would seem to be a limiting factor no matter what coil is used. Seems to me the only way of increasing the peak voltage would be a higher turns ratio or more inductance.
I am always a little skeptical of advertised specs, as they are often derived using “best case” conditions which may not achievable in a normal application.
