Seems a reasonable assumption that lowering IQ is a decent proxy for other damage lead does to the brain, right? It would be weird if it just damaged the bits associated with doing well on IQ tests, and left the rest alone.

E.g. it happens that IQ is something we measure, and EQ much less so. That lead leads to measurable reductions in IQ is strong evidence that it damages EQ too. (As its association with crime would suggest.)