From the article: "The statistical significance of the drop is extremely strong"

I haven't read the source paper yet, but the way this is phrased makes me skeptical of the strength of these findings. Having worked briefly on a lead remediation project for a city government who has been in the news recently for lead, I can tell you that this is a really tough question and the confounding factors are huge. The political consequences mean that folks have incentive to seriously overstate (or understate) the importance and validity of a study like this.