For poverty effects to dominate across countries, there would need to be a causal relationship between phasing out leaded petrol and countries' distribution of income; it looks implausible at first glance. It's much easier to phase out leaded petrol than to make persistent changes in the distribution of income. It would be very surprising if all countries not only did so, but did so at the same time.
Quite, phasing out leaded petrol at different times is the kind of "natural experiment" that can demonstrate the effects of lead reduction not poverty reduction.
This is not the same thing as "poverty drives increased pollution exposure, as a general rule" which is still true.
Other things drive pollution exposure too, like general use of leaded petrol.
