This should be easy to answer: we are not smarter in the recent decades (the opposite is likely true due to the leaded petrol). The natural selection for smartness is too weak in humans (it does not take much brain to reproduce nowadays).
The hypothesis in TFA is much more likely (regular exposure to bright daylight in children)
(Though because IQ is standardized around a mean of 100, it's probably more accurate to say that the IQ of people in the past has retroactively gotten lower)
The flynn effect is well documented. The negative effect of leaded gas on intelligence just means that average IQ "should" have been raising even more throughout the 20th century.
The hypothesis in TFA is much more likely (regular exposure to bright daylight in children)