@Amicchan, Nazism generally refers to the ideology of the Nazi Party and Nazi Germany before and during the Second World War. Whereas Neo-Nazis refer to the modern adherents of that ideology. The term "Alt-right" did not appear in this context until the Cold War and Post-Cold War periods, in which its modern usage refers entirely to Neo-Nazis, among a greater mixture of white-supremacists, Neo-fascists, and radical Trumpists.
It would be ahistorical to call the Nazis of the 1930s and 1940s "alt-right", as that term would only develop in the context of US-American Right-wing extremist groups who were and have been attempting to present their reactionary ideology as the "New/Alternative Right", rather than the "old Right" of the Republican Party.