Meaning of “overman”

By deleting the indefinite article an he's referring to the concept or philosophy: overman. He could have similarly said:

Goethe served Neitzche as a perfect model of fatherhood.

Overman is an unusual example because the philosophy is named in a way that seems like a description of an individual.

Note that in my example above we can't say: a fatherhood. Instead we'd say: a father.

So, yes, he could say:

Goethe served Nietzsche as a perfect model for an overman.

But, the semantic meaning shifts slightly in the same way that substituting a father for fatherhood would in my example sentence.

Side note: übermensch is a well-recognized word to the English speaking audience. It's not clear why the author felt the need to translate it past the initial introduction.