The emissions-per-GDP is probably closely related to technological development and energy efficiency. (At least it seems that way looking at the wikipedia list

The emissions-per-GDP is probably closely related to technological development and energy efficiency. (At least it seems that way looking at the wikipedia list posted, the top countries are all 3rd world nations while for example the UAE is near the bottom.) So, I don't think they've passed the US, I think they've never been below.