Eric Metaxas doesn't have a clue. The Church leaders in Germany at the time were very much married to the idea of Church and the King. In other words, there was no separation between Church and State but they were two sides of the same coin. Even one of the leading figures in the resistance, Martin Niemöller, was very reluctant to let go of the idea that the Church should be subservient to Hitler as King. They weren't cowards, that was just their mindset. Besides, it wasn't the silence of Christians in Germany that led to the Nazis and the Third Reich, it was everyone's silence.
I think Metaxas is just looking for an angle to sell yet another book.