It is most probable that this confidence comes from a complete lack of morals or social awareness, meaning someone with dumbfidence will never question their own actions or opinions. This provides them with the confidence to be a success (...a chicken or the egg scenario).
see also: Footballing Brain
q) How the hell is Boris Johnson mayor of London?
a) he's got Dumbfidence
q) How did someone like John Terry ever become England captain?