You're looking at it as though it's Calvinism. Stalin's statement is about where the ideology of social democracy fits into the framework of class struggle, not about how everyone who at any point identifies as socdem has the soul of a fascist and has never and will never do anything worthwhile. The point of his statement is that the purpose social democracy serves is the maintenance of capitalism and therefore that people working towards social-democratic ends are working on maintaining capitalism (which is also the job of fascism and fascists, though they accomplish it differently and under different circumstances).

That individual socdems radicalized or fought in self-defense or whatever has no bearing on the statement. What matters for it is that the socdem organization of the SPD, on the eve of revolution, chose to protect capitalism (even though many of its own members objected), demonstrating how even those smol bean well-meaning SPD members who just didn't appreciate Rosa's message until her corpse was dumped in the river nonetheless had been working towards the benefit and promotion of just the organization responsible for it.

Stalin joining the allies also has no bearing on this. It fails to comprehend the difference between ideological conflict and political conflict. What Stalin was drawing were the lines of ideological conflict, which is vindicated a thousand times over by the US, Britain, etc. materially supporting Nazi Germany up until the latter's expansionism put it and the various liberal states into political conflict. Because of this political conflict, it made sense to ally in the war with these liberal states, but that by no means made them somehow fellow-travelers, as demonstrated by how the US didn't even wait until the end of the war to start making barbaric plays in the interest of checking the power of the Soviet Union.