The pattern generally reflects the history of immigration. The Protestants were the overwhelming majority of the early immigrants.
Most of slavery happens in a period when Protestantism was virtually the state religion. That explains why there are very few Afram Catholics.
The Catholic wave comes with Irish, Italian, Polish then later Hispanic immigrants. The first three, together with Germans, dominate immigration for most of the last century. They do this to a point where the dominant white stock comes from those regions.
To go to a Catholic Church as a black man in the US is to stand out like a sore thumb. I haven't yet met one who is not a recent immigrant or descended from one.
IQ has been found to have negative correlation to religiosity. I have seen atheist who I would describe as religious. And they quote Richard Dawkins liberally.
But generally, non-believers in the west will tend to be selected from a small pool with higher IQs. That probably explains the observation.
It's really more a statement about diversity than IQ in general. Atheists tend to belong to a similar demographic.
In Africa, I think the converse may be true if you look at the overall population. Most folks who went to school also got a good dose of evangelism.