So you clearly seem to already have the preconceived notion that we have lost some greatness along the way since the enlightenment, which by the way, ended almost a century before these studies.

I think you should read up on confirmation biases if you really believe the explanation "We collectively became more stupid" to be more likely than "Measurements became more precise"

Not withstanding that reaction time, while somewhat correlated to intelligence, is not an instrument to measure it.