This movie didn't age well (whites have become more prominent in boxing since then)
Consider the lengthy heavyweight title reign of Wladimir Klitschko. A case could be made that he ranks alongside Joe Louis, Muhammad Ali and Larry Holmes just based on his longetivity as a champion.
Tyson Fury is the top heavyweight as of this writing.
I suppose that at the time, everybody would have thought that there were no viable white heavyweights considering that from the mid 1960s until the mid 1990s saw no real white heavyweight champions (Gerrie Coetzee held one of the titles in the 1980s for a few months. Tommy Morrison only held the WBO title for a brief time).