No, it isn't. American troops fought the Nazis, after Hitler declared war on the US. They were demonstrably fighting Nazism whilst doing so, and let us not forget that the US agreed to treat Hitler and his regime as the main enemy in the war - giving far more attention to Germany than the Pacific got. So to say that they weren't fighting Nazism, despite three years and thousands of deaths sustained fighting Nazis, is utter nonsense.
Also as Tubey says, they were also fighting unofficially before that, either as individuals (like the Eagle Squadrons of the RAF) or as convoy escorts (at least two of whom got torpedoed prior to December 1941), as well as almost exclusively* backing the UK in terms of munitions and supplies.
* with the exception of IBM, who were backing Hitler