This large current causes the voltage on the vehicle bus to increase significantly—well above the normal and regulated level.
The strength of the spike depends on many factors including the speed at which the alternator is rotating and the current which was being supplied to the battery before it was disconnected.
[1] This kind of a spike would damage many semiconductor devices, e.g. ECUs, that may be connected to the alternator.
That may have a larger voltage, but it will be for a much shorter duration, as relatively little energy is stored in the inductance of these windings.
Load dump can be more damaging because the alternator continues to generate power until the field current can decrease, so much more energy can be released.