I think the article just did a poor job of portraying the rarity of the event. This article links to a Gizmodo article[1] which does a better job of explaining that they had 3.2 tonnes xenon-124 and many occurrences:

> After 214 days of observing (177 days of usable data), the researchers’ analysis revealed approximately 126 two-neutrino double electron capture events.

[1] https://gizmodo.com/dark-matter-detector-makes-incredible-ne...