It's hard to deny that that a hard lockdown similar to the Chinese or the Italian one would have spared some of those lives, but it would have come with other costs (which, as you point, also cost lives).

So the "preventable" is speculative as well, nobody knows what would have happened with a lockdown , but there is an indication: the neighbouring Denmark was having a very fast spread of the disease until they imposed lockdown (which was a political decision) and they have had -until now- a much lower death toll per inhabitant.

But both approaches are reasonable IMHO. The Swedish strategy is based on pessimistic assumptions: we cannot stop the disease and the vaccine will be available late, if ever, therefore let's try to live with it and limit the damage. Herd immunity will eventually stop it. The Danish approach is more optimistic: let's block this now, whatever it costs, and hope that some cure (or a vaccine) comes before the disease starts spreading again.

Which approach will make less victims in the long run? We'll see, but both are bets.