Comparing death rates in Australia, which actually did pretty major lockdowns, to South Africa[1], I don't know that this is a great argument against them. South Africa definitely paid for that infection-based immunity in human lives.

[1] https://ourworldindata.org/explorers/coronavirus-data-explor...

Of course heavy and complete lockdowns are effective (at delaying the spread of the virus) - if no one is in contact with anyone else there's no way for the virus to spread. A more apt comparison would be made between countries/states with slight or "normal/average" lockdown to ones without, but that's very difficult to do in practice. And then measure the financial and societal impacts (which is also difficult/impossible).