Actually this is not correct. The MK21 has the characteristic of a wide cardioid and has some proximity effect. Its a mix between a pressure microphone and a gradient microphone, defined as 1/3 gradient and 2/3 pressure...a supercardioid is defined 63,5 % gradient and 36,5 % pressure.
Read here for more information:
SCHOEPS wide-cardioids
The MK21 has a wide range of use cases... :) I've seen it used as a close mic for piano, as a main mic for orchestra and also for voice.. ;)
greets
ray