There are a few reasons why Americans may have known about events that were not specifically referred to as the Holocaust while it was happening:
1- News reports: While the full extent of the Holocaust was not widely reported until later in the war, news reports from Europe did provide information about Nazi atrocities against Jews and other groups. For example, in 1942, the New York Times reported on mass shootings of Jews in Poland.
2- Eye-witness accounts: Some Americans who were living in Europe or who were involved in the war effort may have witnessed or heard about atrocities firsthand. These accounts could have been shared through letters, telegrams, or other means.
3- Propaganda: The US government used propaganda to raise awareness of the atrocities being committed by the Nazis. Films, posters, and other materials were produced to highlight the evils of the Nazi regime.
4- Prejudice and discrimination: Antisemitism and racism were present in American society during the war, and some people may have been more likely to believe reports of Nazi atrocities because they fit with their existing biases.
Overall, while the full extent of the Holocaust was not widely known until after the war, there were still many sources of information available that could have informed Americans about the atrocities being committed by the Nazis.