“The American Thinker” didn’t think this through very well. The Nazis were socialists, not marxists. Anti-communism, AKA “Jewish Bolshevism”, was Adolph’s big talking point.
Calling socialists Marxists is not a great leap at all. Since Marx was the ultimate teacher of the society over and above the individual, all the variants toward Totalitarianism can accurately be called Marxists.
In the 16th and 17th Centuries, Catholics and Protestants warred with each other—and had some significant religious differences. That didn’t mean they were both not rightly called Christians.