Well, it was, in a manner of speaking. It was on German Army belt buckles and other accoutrements, but it had been there for a long time before the Nazis and the Nazis just never changed it. Proves nothing about the Nazis one way or another.
I wasn't aware of the fact you mentioned. And I can't deny that the Nazis did try to appeal to Christian "tropes," if you will, when it was to their advantage, such as telling soldiers to execute Jews "because the Jews killed Christ." However, as you said, it proves no connection between Nazism and Christianity, any more than the Soviet regime use of the Orthodox Patriarchate to drum up patriotism during WW II proves that the CPSU was really "Christian."