He's saying that Social Democracy and Fascism form the left and right fists of the bourgeois imperialist states. They don't oppose each other from the perspective of those outside the empire and facing it's attacks, they support and regulate each other. Without Social Democracy regulating and sustaining capitalism and mitigating its crises the economic system would collapse and internal crisis would dominate and the empire would be unable to sustain outward pressure. Without Fascism, a social democratic Liberal state would transition into socialism and go too far - so it keeps fascism around to keep the socialists in check. Fascism is the necessary violence of the capitalist state, even a social democratic state, to sustain itself and enforce the limits of democracy (ie, to not allow the poor to vote away the wealth of the rich. There must be a hardline protection against that).

From the perspective of those outside of "Social Democratic Liberal Democracies" facing their imperialism, all they are seeing is fascism being imported into their country and invasions and meddling from fascist attacks. The social democrats keep things stable at home so that the fascist imperialists can extract value from the colonies and beat down any leftists inside the core. The fascists brutally destroy communists and leftists on behalf of capital, while being fed and clothed by the social democrats. It's a symbiotic marriage.