Interesting. It doesn't say what the sample size in the study was.
It is true that far more young people now go on to university and obtain a degree, because there are far more universities and educational standards have fallen a long way from what they were in 1969.
Where there are decent non selective schools, I'm sure there is probably not much of an advantage in going to a grammar school.
However, if the non selective schools in an area are very poor, I think many children would still do better in a grammar school.
This article won't stop parents from going to great lengths to get their children into grammars!

I'd be interested on the results of a study based on the different selective systems.

I would hazard a guess a super selective would garner a different outcome