What do you think ‘super’ means ? It is latin for ‘over’, wich in German is über. In English it has come to take on a broader meaning, but Nietzsche’s übermensch is called ‘superman’ in most English translations, even if ‘superhuman’ would be more accurate.

GP doesn’t imply Nazis used ‘Supermensch’, just that the ‘superhuman’ translates to übermensch and that the branding might evoke this concept for European ears.