When someone says capitalism is human nature, I don’t think they mean that industrial automation allowing unskilled workers is human nature. So they’re using a different meaning of capitalism. To address their concern, you would show counter examples of large groups of people working together for a common good rather than their own enrichment. Rather than just saying they’re using the word wrong.
Do you think you can have effective communism with only self interested parties? That was my take away from your comment, that you can get communism as a logical extension of greedy motives?