Contre Sainte-Beuve précédé de Pastiches et mélanges et suivi de Essais et articles, Pléliade, 1971, p.922 : "In 1906 the first French translation of Tolstoy's essay Shakespeare was published: in it Shakespeare was severely criticized from the point of view of art and particularly of morality. The review Les Lettres immediately invited a number of French writers to say whether or not they approved of Tolstoy's indictment. Proust was one of those writers, but his response was not published. Perhaps he did not send it. Two drafts were found among his papers [...] The journalist who organized the inquiry was called Muller."
Kolb-Proust Archive ID: C1150: 15 August 1907. Reading in the Revue des Deux-Mondes an article setting out Tolstoy's views on Shakespeare, Proust responded to the author, Théodore de Wyzewa, reproaching him for giving "le plus fâcheux retentissement à une héresie" (the most inopportune propagation of a heresy) and "d'ouvrir un schisme douloureux" (opening a painful schism) between Shakespeare and Tolstoy, two great geniuses.
As the expressions quoted by Kolb are identical to those in the Contre Sainte-Beuve article it seems certain that they are the same. The mention of M. Muller in the draft manuscript seems to confirm that this was a response to Les Lettres not La Revue des Deux-Mondes.