The identification of "Prussia" as his place of birth is not inconsistent with this part of the Saarland, as I understand the situation.After the Congress of Vienna, Prussia was awarded lands formerly belonging to the County of Nassau but brought under French control prior to this 1815 redrawing of the map of Europe.This area would have included Remmesweiler.
See the recent history of Prussia by Christopher Clark, "Iron Kingdom," for a discussion of the Prussian acquisition of Rhineland Province, in Chapter 12.Because Russia had taken both a major role and a major "hit" in defeating Napoleon (the failed Moscow campaign!), the treaty transferred Polish territory that had been taken by Prussia in the 1790's over to Russia for her "pain and suffering."To compensate Prussia then, she received in exchange for her Polish Partition lands a great portion of Saxony, part of Pomerania, "and a vast tract of Rhenish and Westphalian territory reaching from Hanover in the east to the Netherlands and France in the west."
Thus, the John Jacob I have in mind would have been born in Prussia, and Prussia would still have been in charge of his home village at the time of the census of 1870.