To say diversity is strength isn't a mere meme or pretty saying. It's a fact. Genetically speaking a heterogeneous line is a healthy one; a homogenous one presages extinction. Culturally speaking a diverse one is more mature, secure in itself, while a homologous one is insecure, defensive, and prey to undoing from without and within.
Dreams of cultural uniformity are also just that: dreams. There's no such thing as a homologous or uniform culture. Every category much less country is multiple, even ones that have the appearance of cultural or genetic homogeny. Get to know the history of any nationality, for instance, and one finds centuries and even millenniums of ferment between different, shifting, often marauding groups with varying origins, ones that in most cases were stilled, but usually bullied into submission by the imposition of the modern nation state, as is the case in most European countries, that or imposed from outside, usually colonial forces, as is the case of many Arab and African states.
In The Lede the white-flight community of Laurentine represents allegorically that element in American culture that fancies itself a white nation. It's a bastion with a bastion mentality––or to use a Dutch metaphor Harry has his finger in the dike to hold the deluge of diversity at bay. The result is rot from inside. In fact the family is the true allegory for the nation in the novel; it disintegrates from within because of the race-panic of the parents, and the father specifically. The twins don't share the old generation's fears. They embrace openness, even if they have a long way to go in discovering what it really might mean, the responsibilities it entails, as well as what it might offer.
They learn they need to rethink many of the things they were taught. They'll discover whiteness is a myth for one; that race is generally; that there's only one race among human animals. They'll discover too that all binarisms, including black/white, are structures that maintain some old, outdated, and unjust power imbalance.
They'll also learn that diversity is a cultural wealth, and they're more content than their parents as a result. Living without race fears and free from a bastion mentality, the twins' lives will be many times richer than that of their parents. They can love anyone, discover new worlds literally and metaphorically, and acquire new vocabularies. A world––the world––several worlds––open up to them, not to exploit or control, as whites have too often done in the past, but to honor, respect, and celebrate.