The harm by lead occurs in the first 3 years of life. High blood lead in early childhood is not detected later in childhood [0]. The New Zealand study uses blood tests performed ten years too late. This flaw makes the study almost useless.

Additionally, only 30 of the children in the study had low blood lead, <5 ug/dL. This is because the blood was collected in the 1980s and New Zealand only banned leaded gasoline in 1996. Look at Figure 1 in [1], and notice that at 5ug/dL, the damage is already done.

[0] Caito, S., & Aschner, M. (2017). Developmental Neurotoxicity of Lead. Neurotoxicity of Metals, 3–12. doi:10.1007/978-3-319-60189-2_1

[1] Nevin, R. (2000). How Lead Exposure Relates to Temporal Changes in IQ, Violent Crime, and Unwed Pregnancy. Environmental Research, 83(1), 1–22. doi:10.1006/enrs.1999.4045