I don't think they mean humanist in the sense of atheism, actually, but in some sort of social communitarian sense. The Muffins are not Mennonites. No-where in the Muffin Manifesto ("massively bourgeoise and selfish" indeed) is there any mention of charity as a virtue these women might possess, or any sort of social contract - in a sense they are extreme Thatcherites: "there's no such thing as society. There are individual men and women and there are families" - whereas Alcott brims with ideas of mutual responsibility, Christian charity etc. One does not imagine the Muffin daughter coming home to say "Daddy! The poor immigrant family down the road have no Christmas dinner. Let us give them ours."