I suspect so. Mendelsohn was a practicing Jew who did, for example, keep the laws re: food to the letter, and who later was claimed by both the orthodox and the reform movement in German-Jewish circles; however, he did also speak against some parts of Jewish orthodoxy, such as saying that the rabbinate should not have the right to punish Jews deviating form orthodoxy by legal measures (such as the expellation discussed above), and statements like Mendelsohn's saying that Judaism was less a "divine need, than a revealed life" were why later, younger German-Jewish writers like Heine saw him as a kind of Luther figure. However, I can see D'Argens making assumptions simply because Mendelssohn was an enlightenment philosopher. Or maybe he thought it would play better with Fritz than explaining Mendelsohn's more complicated brand of faith?

Btw, I assume you know about Moses' grandson Felix the composer, but do you know about his granddaughter Fanny, Felix' sister, also a composer? (And an illustration as to why the "there were no female Mozarts, were there?" argument is stupid, because Fanny is a rl illustration to Virginia Woolf's "Shakespeare's sister" essay?