Memes

That's not what the other commenter is speaking about, the speaker (and this meme) are talking about these concepts in the Marxist sense. Idealism is closer to the idea that ideas exist independent of surrounding reality, and an application of idealism would be the assertion that Marx was an especially gifted human that came up with Marxism of his own. Materialism asserts instead that Marx existed within the context of his existence, and his experiences and those he learned from were the primary genesis of Marxism.

Marx asserts that Materialism is true. In the context of the prior example, Marxism could not have come about before society had learned and advanced to the level that Marx first was born in. Marxism may not have come from Marx, but anyone else following that period, who had similar material conditions, but the prerequisite progression of society and the experiences before them allowed Marxism to come to being.

I recommend reading Elementary Principles of Philosophy to better understand these concepts.