Do and La are different
Rousseau is referring to the fact that sol-fa note designations are based on the relative position of the notes in each key's scale rather than on absolute notes of music.
Rousseau himself was the victim of a false accusation of plagiarism because of a misunderstanding of this distinction. When he first arrived in Paris in 1742 he had hoped to gain fame and fortune with a system of numerical music notation that he had invented to make the reading of music easier and had gained a hearing with the French Academy of Sciences to present his system for approval. The commissioners termed the system "unoriginal" and cited a system of notation devised by Souhaitty a century earlier. Sidney Kleinman has recently shown, however, that Souhaitty's system was based on a fixed note progression whereas Rousseau's was based on a key-related progression. In the end Rousseau's system was "reinvented" in the 19th century and became the one used to teach music to young people in China. (See"Rousseau et Souhaitty: Une Fausse Accusation de Plagiat" inGuy Lafrance, ed., Studies on the Social Contract (Ottawa: North American Association for the Study of Rousseau, 1989).