It's not exactly what it does say. Firstly for some reason the original research link is broken. Secondly (from your own source):
"Nonetheless, for the teens and young adults of the late 1970s, the economy seems to have been, in at least some meaningful sense, a meritocracy.
Now about that catch. The unfortunate truth is that, more often than not, the rich kids are the smart kids. For many years now, the single biggest gap in American education has been between the well-to-do and the poor. Thanks to the resources their families can pour into parenting, wealthy students start out academically ahead the day they walk into kindergarden, and stay ahead through their high school graduation day."
Lastly I'll add that it only took white non hispanics into account.
"Nonetheless, for the teens and young adults of the late 1970s, the economy seems to have been, in at least some meaningful sense, a meritocracy. Now about that catch. The unfortunate truth is that, more often than not, the rich kids are the smart kids. For many years now, the single biggest gap in American education has been between the well-to-do and the poor. Thanks to the resources their families can pour into parenting, wealthy students start out academically ahead the day they walk into kindergarden, and stay ahead through their high school graduation day."
Lastly I'll add that it only took white non hispanics into account.