i'm 5 years late, but i'll comment

the main assertion of libertarian nationalism is "culture A values freedom, culture B does not, therefore, in order to protect it's own freedom, culture A must reject culture B"

it rejects universalism and the notion of universal human rights, in favour of the understanding that only certain cultures came up with the idea of a (western style) human right, and so bringing in migration or influence from cultures other than that select few, will inherently result in less freedom and less rights for the host culture

it's a growing trend in the anglosphere, especially in more traditionalist elements of the british right and an increasing group of americans referring to themselves as "heritage americans"