Fascism is the bourgeoisie’s fighting organisation that relies on the active support of Social-Democracy. Social-Democracy is objectively the moderate wing of fascism. There is no ground for assuming that the fighting organisation of the bourgeoisie can achieve decisive successes in battles, or in governing the country, without the active support of Social-Democracy. There is just as little ground for thinking that Social-Democracy can achieve decisive successes in battles, or in governing the country, without the active support of the fighting organisation of the bourgeoisie. These organisations do not negate, but supplement each other. They are not antipodes, they are twins. Fascism is an informal political bloc of these two chief organisations; a bloc, which arose in the circumstances of the post-war crisis of imperialism, and which is intended for combating the proletarian revolution.

Source: Section 1 of “Concerning the International Situation,” Works, Vol. 6, January-November, 1924, pp. 293-314.

This is what this person means, this is what you need to understand. It means not that they are not different, but that they are too the same thing under different light.