I'm not a fan of Eco's 14 points as an identification of fascism. They are common characteristics of fascism, but not the definition of it nor do they explain the rise and fall of fascism. Fascism is better seen as capitalism in decay, as the methods of colonialism applied domestically. The Nazis essentially tried to colonize Europe, treating the rest of Europe the way Europe treats the global south.

Social fascism is essentially a term for those that want to retain imperialist welfare states, bribing the working classes with welfare and siding with imperialism and neocolonialism to fund them, rather than socialism. The German SPD hated the communists, and worked with the Nazis to kill the KPD off, enabling the rise of Nazism in Germany. It's no coincidence that PJ loves the German SPD.