It is not, and you are using a fallacious slippery slope argument to double down on a really bizarre and utterly offensive interpretation of altruism.
While I have trouble understanding his actual point to be honest, I think he might be operating under the idea that he wouldn't be able to choose to be altruistic in case of a Good victory, because he'd be pressured into being altruistic. Though being pressured into it would make it not altruism, so it would be self-defeating for Good people to do that, or it would be distorting the term a lot -- like paying for what you buy isn't altruism, even if one could concievably run away with the goods instead.
If you think that choosing to act kindly towards others is the same as being their legal property, then you need to do some learning about the actual history and horrifying human experience of slavery.
On the other hand, let's not reduce the actual history of slavery to the chattel slavery of the colonial triangular trade and the American cotton farming history, which was probably one of the worst case possible. While it might be the first association made in the US and in places which benefitted from triangular trade, it might not be the one that the person you're debating with makes. Many Roman slaves where actually better off than poor free men, leading centuries later to the disappearance of slavery in the Eastern Roman Empire because hiring laborers was actually less expensive for large landowners than the slave upkeep, so freeing slave was actually economically sound. While it is certainly horrible to be a eunuch at the Qin court, it's better than be a toiling peasant, or drafted into working to death to build the Great Wall, despite not being a slave, and eunuch became, during the later Tang and Ming dynasties, and despite the cultural stigma of castration (which adds but is quite distinct from slavery), a coveted position.
The person mentionning slave doesn't necessary think of the worst case of slavery you're referring to (FWIW, I think of Ancient Greece/Rome foremost, then Egypt even though the pyramid builders weren't actual slave, but it's an association I made as a kid, then of the Atlantic slave trade only), much like when Nietzsche writes about "slave morality", he doesn't refer to actually owned people, but to any kind of person psychologically in a state of submission. I think a Nietzschean reading is much more coherent with the idea developped by the poster you're responding to (at least, it makes it not nonsensical).
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