The "dictatorship of the proletariat" is mostly Marx doing what we would now call "being an edge lord". It's supposed to be a contradiction of the "dictatorship of capital" or rule by the owning class. But it's all edge and no point because the central argument is an abolition of the legal concept of capital, literally abolishing the thing that would allow capitalists to continue to exist a distinct group the proletariat could "rule over".
The thing that is unconstitutional isn't the "dictatorship of the proletariat" by abolition of capital, it's the oppressive authoritarian rule that has in the past and present been justified with appeals to that kind of language, especially in Marxist-Leninist schools of thought. This is relevant because the likely frame of reference for the court here is the German Democratic Republic, the former vassal state of the Soviet Union. But even they opposed a close reading of Marxism which they argued was "utopianist", instead justifying the supposed inevitability of their totalitarian rule as "real existing socialism" rather than communism.
Marx did not argue for a ruling bureaucrat class to replace the capital owning class. It's easy to see how that kind of thought would be at odds with the constitution but even most "left wing extremist" groups in Germany are ideologically opposed to it, Marxist or otherwise.