How did Marx distinguish communism from other similar theories?

Karl Marx and Friedrich Engels obviously believe that practical action is far more significant than theoretical thought. This idea is powerful and well-defined in The Communist Manifesto. It is this concept which colors Marx’s rejection of anything other than his own revolutionary declarations. In The Communist Manifesto, this concept is applied to denunciations of other “Social and Communist literature” and to the call for all workers of the world to unite.
In the third section of The Communist Manifesto, Marx and Engels reject other socialist doctrines to distinguish their own Communism from these other literatures. These other socialist ideologies do not recognize the paramount role of the proletariat, and that is a significant reason why they are dismissed by Marx and Engels. However, the concept of necessary revolutionary action seems to be also prominent. Describing “Critical-Utopian Socialism and Communism,” they preeminently stress that this doctrine “reject[s] all political, and especially all revolutionary, action.” Marx and Engels describe this type of thinking as “a fantastic conception.” According to them, revolutionary action is only thing that suffices to make fundamental changes in society, politics, and the economy. The socialists who “violently oppose all political action on the part of the working class” are idle dreamers who will never progressively alter the course of history. Thus, their idealistic fantasies, unsupported by real action, are “necessarily doomed to failure.” Marx and Engels reject, too, the idea of gradual reform. “Conservative, or Bourgeois, Socialism” does not understand “abolition of the bourgeois relations of production, an abolition that can be effected only by a revolution.” These “administrative reforms,” gradual steps of socialism rather than complete revolution, actually do not “affect the relations between capital and labour” at all. In fact, this redressing of social grievances is only “in order to secure the continued existence of bourgeois society.” Accordingly, Marx and Engels maintain that Communist revolution is the only way to bring about the resolution to class struggle.
The implications of this claim are vast. It is easy to see how this argument makes sense philosophically because revolutionary action brings about change in a manner that no other action or inaction can. This concept also brings a political question to the surface – how would this revolutionary action be realized, realistically and tangibly? It is easy to grasp as a theory, but Marx himself denounces theory without action.
Marx’s argument is strong and clear; he characterizes his perspective in a manner that is practical. What is interesting is how Communism seems to be characterized, mostly by modern popular media, as overly optimistic and therefore unrealistic. Yet, The Communist Manifesto clearly distinguishes Marx’s Communism from the types of socialism that are actually overly optimistic and unworkable. These other “castles in the air” are categorically discarded by Marx.