This website of course has its own biases, and I found its arguments suspect and unconvincing. Consider for example the objection, “Nowhere does Hitler denounce Jesus or his own brand of Christianity.” Would Hitler ever denounce “his own brand of Christianity” if he was not a Christian? Furthermore, what about Hitler’s claim that the Christian doctrine will be convicted of absurdity? The author never mentions that tidbit. H.R. Trevor-Roper is a distinguished historian, and I trust his judgment on the authenticity of the records (see e.g. page xiv of the book) more than an atheist on the Internet who wants to believe Hitler was a Christian.
Certainly there are some atheists on the Internet who question book’s authenticity. There are also atheists on the Internet who argue that Jesus of Nazareth never existed (mainstream scholarship be damned).