Unclear. The cohort most exposed to atmospheric lead was born between 1951 and 1980 [1]. This study only measures those born before 1948.
(To the extent the study supports a hypothesis, it's the null. Given atmospheric lead increased from 1890 onwards, until about the 1980s [2], if lead were the culprit we'd expect to see more dementia among the study's younger generations. Not less.)