I believe he's trying to make the point that it is misleading to claim affiliation to a certain ethnic group unless you have at least 50% of genetic affinity with said group. Of course, such argument is fallacious on the basis that is simply not how ethnic affiliation works or has always worked in human history.
Take for instance Greeks and Turks, these historical enemies. Both have a mere 10% genetic similarity with their Proto-Indo-European and Proto-Turkic predecessors, and yet their entire culture, language etc. has been defined by these minorities for centuries or even millenia. Same for the population of the United States, of which less than 50% descend from Americans living there a mere century ago. Or the English, who believed they were pure Anglo-Saxons, but actually are 50% Insular Celts, which in turn were only 50% Proto-Indo-European...
I assure you he's not putting any kind of thoughtful numbers behind it.
Massimus - "Trump already has democrat support."