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.

Sorry, I thought it was a thinly-veiled moral argument equating bitcoin mining to use of slaves. Glad I was wrong.

Correct.