For their collective disunity in the face of evil, Martin Niemöller along with all other believers in lands occupied by the 3rd Reich were thrown into Satan's sieve to see who was wheat and who was chaff.
The German electorate in 1932 and 1933 was deeply divided. Practicing Catholics wouldn't vote for a Protestant candidate and Practicing Protestants wouldn't vote for a Catholic candidate. Because of their long history of mistreatment, Jews wouldn't vote for either. The end result was Hitler.
We got a glimpse of that here on FR in the runup to the 2012 presidential election with posters making "principled" stands to not vote for a Mormon.
Paul von Hindenburg, who defeated Hitler twice in the 1932 German Presidential election, was Lutheran. Perhaps it is fair to assume, since German Protestants outnumbered German Catholics by about two to one at that time, that your postulation is correct.
However, I'll need some convincing that religious differences among the German electorate led to Hindenburg appointing Hitler as Chancellor of Germany. As you know, Hitler never actually won a free election while standing personally as a candidate, and even at the time of his accession to power the Nazis were the largest party in the Reichstag but not an outright majority.
But yes, the Stuttgart declaration is an argument that Niemoller accepted personal spiritual responsibility for his past actions. Wheat and chaff separated but at a frightful cost.