Form the article: >The deviation presented today is consistent with a pattern of anomalies measured in similar processes by LHCb and other experiments worldwide

Form the article: >The deviation presented today is consistent with a pattern of anomalies measured in similar processes by LHCb and other experiments worldwide over the past decade. So while you're correct that the 3 sigma is quite a low significance by particle physics standards, it is consistent with previous measurements which yielded similar anomalies, presumably at lower significance.