I haven't read the book, but it seems to me as if you're making this judgement based solely on the line "an idea as compelling to the Greek and Roman stoics" an

I haven't read the book, but it seems to me as if you're making this judgement based solely on the line "an idea as compelling to the Greek and Roman stoics" and ignoring the fact that it's immediately followed up by "as to Sartre and Camus". Ignoring what you said about Stoicism (which,quite frankly, is wrong; it's a very humanistic and community oriented philosophy and almost all virtue-based philosophies are action oriented) the idea of detachment is common to many philosophies and religions including Buddhism, Absurdism, Nihilism (IMHO), various monastic and mystic groups (i.e. The Church Fathers and Sufism), and good old fashioned Platonism. Stocism was probably just mentioned since it A) does incorporate detachment to some degree and B) is one of if not the most well known of the Greco-Roman philosophical schools.