To impute the effect of lead, you look at a bunch of people, measure the amount of lead in their blood, measure their IQs, and see how much of a difference there is between people with a lot of lead and people with less or none.
Modern poor people who live in crummy areas where there's still a little bit of lead are about as stupid, relative to the leadless elite, as poor people from decades past who lived in crummy areas which, at the time, had a lot more lead than they do now.
It seems like a safe assumption that the effect of lead on people with negligible lead levels has stayed constant over the decades at indistinguishable-from-zero.
But for lead to explain the gap between the lead-haves and the lead-have-nots, its effect must have increased dramatically over that same period. That gap hasn't changed. But lead levels have plummeted.