Both Hiroshima and Nagasaki are thriving cities today.

That contradicts the whole point that a nuke will destroy humans but leave the environment intact. A bomb of any kind destroys ecosystems. If humans reclaim the cities, it's not a "net positive" for the environment, despite the cynicism that's in the statement.

"Land back" is a much better approach since land under indigenous jurisdiction has much more biodiversity than average and especially than bombed land.