When a new European state was formed that desired to be a monarchy, it was common to ask a scion of some established royal family to sit on the newly created throne, as this would lend legitimacy to a previously non-existent royal line (other countries will take you more seriously if your new ruler is a member of the respected Bourbon dynasty than some random elevated native).

Some examples of this are Spain asking an Italian Savoyard prince to become king in the 1870s after the first expulsion of the Bourbons, or Mexican conservatives inviting the Hapsburg Maximilian to sit on the Imperial throne in the 1860s, or Greece doing the same with a German prince in 1832

Asking German nobles or royals was particularly common, because Germany was famously fragmented until the 1870 unification into several (at least nominally) independent states including innumerable powerless duchies and principalities whose only real prestige came from housing a noble family. So Germany's many little states provided a wellspring of listless princes and princesses without much to do who might jump at the chance to go be king of some newly created country.

I haven't read this particular book, but I assume McMillian is talking about the German imperial plan to carve new puppet states (Poland, Baltics, Ukraine, etc.) out of territory captured from Russia and sit German princes (even after the unification most German royals like the Wittelsbachs of Bavaria maintained their thrones though they were subservient to the Hohenzollerns) on their thrones.