What distinction does O'Connell make about the Zone and why is the observation ironic?

While discussing the location called the Zone in the Ukraine, O'Connell states that the site of the nuclear disaster is now effectively "the largest nature preserve in all of Europe" (192). Even though the site is still heavily radioactive, the absence of humans has in some respects returned the flora, fauna, and ecosystem as a whole to a level of health unseen for decades before the disaster. The observation is an ironic one and it seeks to point out the unrelenting and destructive nature of the human race in terms of our effect upon our surroundings.