Also, King Frederick II of Prussia was not “German Emperor” (The English subtitles do call him this at several points, but they are often sloppy, and even when the translation is correct the show itself is not quite accurate on some details – the spoken dialogue at times wrongly calls him “Kaiser” rather than König – so this may be the origin of your confusion). Frederick II did have have grandiose ambitions and may have dreamt of achieving the exalted status of Emperor, but he was not (Indeed, his pomposity and megalomania were the origin of the drive toward Prussian/Hohenzollern dominance of intra-German affairs, at the expense of Austria and the Habsburgs, that had such disastrous consequences in the 19th and 20th centuries).

Prussia at the time of Frederick II was actually a parvenu Kingdom that had only been raised to that royal status in 1701 (upon the union of the Margravate of Brandenburg and the Duchy of Prussia). Insofar as there was a “German Emperor” at the time, it was Holy Roman Emperor Franz I (though his wife Empress Maria Theresia was the de facto power really in charge). Formally, Fredrick II was a vassal of the Holy Roman Emperor.