That’s exactly was Nazism is: Nationalsozialismus = National Socialsm.
They were socialist, yes, but also nationalists, and to the right of the hard-core commies.
So, during and after WII, the lefties (and international communism sympathizers) who did not want to be besmirched by the Nazi connotations called them “far right”. And here we are.
Mostly they were upset the fascists betrayed them. Hitler betrayed his contract with Stalin. Otherwise they were pals before that.