That is a message to the people of the world, but it is not the real answer because they are people of the city of man and not people of the City of God. The truth is that they should not have children so that they disappear along with all the others of the world, for they are not fit to raise children.
The real answer to them is that unless they return to the true faith and really live it, they do not stand a chance against Satan and his affiliates. There is no other answer, there is no "middle of the road" solution.
What sweeping generalizations (demonizations?) you make!
There is nothing wrong in being proud of any true good in your racial heritage as well as your national heritage and any proper and valid connection between the two. It doesn't necessarily mean you contemn other races any more than you want your race to be contemned by Jєωιѕн Supremacists or any other group. Any movement (including that the White Nationalist one) can be subject to all kinds of errors, but that does not mean that we should simply out of hand condemn the movement as opposed to trying to correct or influence it for the good via the application of Catholic principles.
Please realize that the Church does not condemn temporal movements simply because they are not in strict accord with every fundamental Catholic Principle. The Vatican via Pope Pius IX was the first government in the world to recognize the South when it exercised its constitutional right to secede from the Union. The Pope even sent a hand made crown of thorns to the Confederacy's President Jefferson Davis when he was held in chains in a Northern prison cell. (Davis and the South were demonized in history books and still are in many ways that White Nationalists are today. By the way, Jєωs -- and this is a well docuмented fact, albeit a very suppressed one -- were much, much more likely to own slaves be it in the South and the North than Whites were before, during, and after the cινιℓ ωαr.)
You make the astounding statement that "they should not have children so that they disappear along with all the others of the world, for they are not fit to raise children." Frankly, that sounds like an assertion from hell. It certainly runs contrary to any kind of Christian principle. How do you propose that "they should not have children?" By contraception? By abortion? By infanticide?
You say the answer lies in them returning to the true faith. Many were never in the true faith in the first place so they certainly can't return to it, although they hopefully can convert. Regardless of whether one belongs to the true faith or not they have a natural right to procreate if they are validly married and yes zillions of people are validly married even though they are not of the true faith.
My point in the OP was simply to point out a great error in the White Nationalist movement which is largely ignored. Yes, short of invincible ignorance preventing them from making a conscious and knowing decision to come into the Church they are condemned to hell, but if they would stop contracepting they should have a better chance of being receptive to being converted to the True Faith and if they are already in the Church and they stop contracepting then they will stand a better chance of growing strong in the faith as opposed to continuing to live in an objective and very possibly a subjective state of mortal sin.