It is terribly difficult to not be guilty of presentism when reading books like this.
The lecture, presented at Tulane University in 1885, was the author's rather scathing endictment of George Washington Cable's 1880 novel entitled The Grandissimes: A Story of Creole Life. What Gayarré takes exception to more than anything else is Cable's presentation of Louisiana Creoles being of mixed blood; the lecture's sole purpose is to say that Creoles were inherently white people. The author maintains that the "inferior" people of color could not have created such a vibrant culture and city.
Historically speaking, the fact that the Vieux Carré (as opposed to the American sector) was one where black and white people lived side by side, if not always in harmony, is what has created the New Orleans of both history and the present day. However, Gayarré must be seen as both a product of his time and a progenitor of the modern-day racism we see on display in current events.