Actually what they're saying is that most people did not have a lot of nurturing adults in their lives growing up, from which they've learned that the adult thing to do is to isolate themselves and avoid nurturing other people. Which they're presenting as a bad thing. The reason they bring up the "nuclear family" is an appeal to prehistory, in which we probably would have been raised by a much larger social group than mom and dad, to include other relatives and friends, and that this would give us more opportunities to be included and nurtured by a wide variety of people.