Yeah, there was a mistake all right. The mistake, though, was the one Puzo made in not naming the sixth family.

One way or another, this discussion is really all about sloppy writing, because there are only two possibilities.

Either Puzo intended that there be 5 families in addition to the Corleones and, sloppily failed to name the fifth.

OR

He intended the five familes to include the Corleones, and sloppily wrote passages such as the one cited above (and others), which strongly implied that there were six families in total.

Either way, it's a mistake by Puzo. I think everyone would, or should, agree with that.

So now the question becomes "Which is more likely to be a mistake"?

Since there were a number of instances in the novel in which Puzo implies that there is a 6th family, and only one occasion where he has the opportunity to name it and doesn't, I have to believe that the weight of the evidence is on my side.

Perhaps his description of the 6th family Don was weak, and edited from the novel. In the book, Michael does not kill the heads of all the other families, so perhaps it was felt that naming them all was unnecessary.

My theory is that since there actually were (and are) five NY families, by including the Corleones as one of them, Puzo felt that he would be inviting too much comparison and speculation as to which of the five the Corleone family was meant to represent. Since the Corleones were the main characters, I believe that would have been the case. No one would have cared, for example, who Cuneo or Stracci was supposed to be modeled after, but everyone would have been trying to figure out which family head Don Corleone was supposed to be.

I believe Puzo simply erred in not naming the sixth family the one time he had the opportunity to do so.