That one was on purpose:

In the book, just like in the movie, Spearchucker reveals that the nickname “Spearchucker”, ordinarily a racial slur, referred in this case to his javelin-throwing prowess.

They use the racism behind his nickname to address racism:

The film also depicts Spearchucker as a particularly skilled neurosurgeon and this, together with the experience of playing football together, helps Duke Forrest overcome his racial prejudices and the two become friends at the end, shaking hands when it is time for Duke to go back stateside.