This is partially because in-school friends are more likely to be achievement-oriented and share and support school-related activities, including studying, because they are all in the same environment.I'd say the out-side school friends are of two kinds, those in different schools and those not in any school. For the first group, why would going to the same school rather than a different school make a kid more or less achievement oriented? Wouldn't each school then be more achievement oriented than the other?
For friends who are still in school compared with those who are not in any school, whether because they're older or because they have dropped out, you would expect this result because of engagement.
But I think this guy did his research in gifted programs. I know some schools where you'd have to search to find achievement oriented kids.
So I don't doubt that the researcher got his data right and so on, but I also doubt that it tells a parent anything about how to raise their kid.