The car was undoubtedly named after René-Robert Cavelier, Sieur de La Salle, a Frenchman who explored the Midwest in the seventeenth century. Similarly, Chrysler named a car after Hernando de Soto, who explored the area a century earlier.
Incidentally, we had a 1946 de Soto, which was a great vehicle. After the the de Soto model was discontinued in 1961, the dealership in Whittier, Calif. closed, but for years afterwards, Hernando gazed over the empty showroom, his bust illuminated by a tubular neon light.
Makes sense especially considering their Detroit (or should I say “de twah”) roots.