This is a very thorough article. As is always best with scientific reporting, the article ends with a prediction and method to verify results. In this case, the prediction is that iron-60 would not be found in ice core samples older than 40,000 years, as at that time the solar system had not yet entered the gas cloud that it is currently in.
I wish they had clarified how they eliminated terrestrial nuclear reactors/devices as possible sources.
I was under the impression this was because the ice sample was 40,000 years old?