But that implies that the members of the party are actually part of the proletarian class. This supposed class membership is purely idealist.

You are incorrectly defining class. This essence of class is: who controls the means of production, and who decides what to do with the surplus value created, and who it goes to benefit.

State administrators (uncharitably bureaucrats as you would call them) are not a class. They do not privately owns means of production, nor collect the surplus value of workers, deciding privately what to do with it. What to do with the surplus, and production decisions, are determined by decision making at the collective, political-level, not at the private level. This means they are not driven by profit motives, since increasing exploitation personally gains them nothing.