Memes

Technically, it's built on the idea that a socialist society can be/should be reached gradually by participating in parliamentary liberal political system instead of overthrowing liberal society and implementing a "dictatorship of the proletariat".

You are mixing social democrats with democratic socialists. Democratic socialists, however ineffective or utopian, at least retain socialist aims in theory. Social democrats do not. Their program, accepts the permanence of capitalist property relations. Their project is not the abolition of exploitation but its rationalization: a "fairer" distribution of imperial superprofits among the labor aristocracy of the core. This is not a path to socialism. It is a management strategy for capitalism.

The meme is clearly pointing out that "social democracy enjoyers" turn into fascists/Nazis once the economy declines. Or, if we keep OP's caption in mind, the idea that social democrats are actually fascists "wearing a mask".

The social democrat's mask, like the liberal's, depends entirely on the surplus extracted from the periphery. When that flow contracts, the mask comes off. In the words of Malcolm X on a similar issue: "The white conservatives aren't friends of the Negro either, but they at least don't try to hide it. They are like wolves; they show their teeth in a snarl that keeps the Negro always aware of where he stands with them. But the white liberals are foxes, who also show their teeth to the Negro but pretend that they are smiling." Social democracy operates the same way. Its niceties are financed by imperial rent. When the rent falls, it defaults to open class defense.

What helped Hitler seize power was not just the actions/inactions of the socdems and the economic collapse, but the deep split of the left overall, the ineffective political system and the relentless infighting to the point were socdems and communists saw eachother as equivalent or even a bigger threat than the fascists.

I explicitly said "helped," not "solely responsible." Multiple factors converged in 1933. But the SPD's role was decisive in one key respect: they preserved the bourgeois state apparatus after 1918. Through the Ebert-Groener pact, they kept the reactionary judiciary, the imperial officer corps, and the bureaucratic machinery intact. They unleashed the Freikorps on the KPD. They refused every proposal for a united working class front against the Nazis. Stalin characterized this relationship precisely when he stated that "Social-Democracy is objectively the moderate wing of fascism" and that these organizations "are not antipodes, they are twins." The KPD's analysis recognized that in a crisis, social democracy functions as the left wing of counterrevolution. History confirmed that analysis.