"Nordicism", particularly as the term is thrown around by Rienzi- and diabloblanco-types, encompasses various distinct schools of thought with divergent purposes, emphases, and interpretations. Here, I'll simply define "Nordicist" as someone who recognizes the existence within Europe of racially distinct type(s) associated today primarily with Northern Europe and her offshoots and who favors the continued existence of these type(s).
I'll let Frank Salter--from whom GW borrowed the term--define "ethny":
A population sharing common descent. 'Ethny' is a preferable term to 'ethnic group' because members of such a category rarely form a group. Ethnies are usually concentric clusters of encompassing populations, such as tribe, regional population, and geographic race. The term 'ethny' used in this book usually means 'a named human population with myths of common ancestry, shared historical memories, one or more elements of common culture, a link with a homeland and a sense of solidarity among at least some of its members'.2 However, it sometimes has a more general meaning, and thus corresponds most closely to the biological concept of the population.
[From On Genetic Interests, Appendix 2; my emphasis.]
What should be immediately clear is that not only is "Nordicism" compatible with the preservation of "ethnies", to the extent the Nordicist correctly discerns relative kinship, Nordicism is synonymous with a desire to preserve a particular ethny or ethnies.
I presume when GW writes "ethny" he has in mind traditionally-defined ethnicities or nationalities. Frank Salter, as we see above, sometimes uses "ethny" in a similar manner. And indeed we should expect present-day national and linguistic borders to play some role in structuring genetic variation in Europe; but it's an empirical question (yet to be investigated adequately) how large that role is. Sir Arthur Keith argued nations are "races in the making" and I agree--but that process of race formation is, I think, far from complete in most European polities. Some "Nordicists" have critiqued elements of nationalism (e.g. "fratricidal" wars), but none of the major Nordicist thinkers I'm aware of ever called for abolition of national borders, customs, languages, or so forth--nor are they desirous only of "preservation at a combinative level." In fact, the ethnonationalist is more like the "pan-Europeanist" in that respect. The "Nordicist" almost by definition recognizes and values the existence of racial variation within national borders. The single-minded nationalist, on the other hand, would seek to minimize regional and class differences to promote national unity (at the likely expense of regional genetic interests). The "Pan-Europeanist" merely expands that mindset to cover an entire continent.
GW's belief that America is not a "real" country is again on display. I'll probably have more to say about that later, but for now I'll note that the genetic distance between SW and NE Germans (FST = 0.00054) is five times that between Americans reporting "western European" ancestry and those reporting "central European" ancestry (FST = 0.0001). No doubt the "ancient" nation of Italy (est. 1861) is even more homogenous.