You are diminishing socialism to an economic system and kind of erasing its long history of philosophical contributions from various socialist thinkers. At it's core it is an economic system supported by an ideology that could allow that system to function properly, much like capitalism has its economic components backed by an ideological framework (ala the 18th century propaganda "greed is good" and its social impact on cultural thinking at large, and how this philosophy allows capitalism to function within society), and one component of ideological socialism is that humans have, in some way, a certain amount of altruism and naturally seek to work collectively. One could say that we're experiencing the cultural shift currently as liberal publications continuously raise issues of class consciousness, greed, and the merits of being a more empathetic human being. This is socialist propaganda in the sense that it will create the cultural thinking that will allow socialism to become more palatable to, and ultimately work within, society. It will also breed the same kind of support for socialism that we see now for capitalism.

I think they were trying to say that capitalism originally had egalitarian ideals, and this is true looking back on the literature of capitalists in the 18th century, many believed that capitalism would end the hierarchical class structure of monarchist society, and allow everyone to achieve social, political, and economic parity. They were wrong, and subsequently many philosophers started reacting to the failures of capitalism immediately afterwards. I think it would have been more accurate to say pro-capitalist was once a firm leftist position.