That's a good point -- in earlier times slaves worked the mines to extract gold from raw earth. Gold was a good store of value because of its scarcity. Likewise, Bitcoin mines the electrical grid.
Are you arguing that using the electricity you paid for is immoral, or are you arguing that using slaves is not immoral as long as you use them for something good?
Slaves are unimportant to his point. Gold being scarce means it was hard to counterfeit gold coins in a time when you couldn't mint coins in a reliable way. Likewise, the purpose of bitcoin mining is to verify transactions in a way that is hard to counterfeit, making it more difficult to get a majority share of mining. The difficulty enforces scarcity.
