The two population cohorts are 11-year olds and 17-year olds so regardless of what they say, we're still just measuring shoe size.
I'm guessing they published these two groups because their adult cohorts showed zero difference and that's not a publishable paper.
In fact, they admit it
"Two previous studies taking this approach, each with a sample size of about 35 sibling pairs, failed to find a significant correlation"
Right before they say "chill, let's cheat":
"We reexamined this empirical issue in a Human Connectome Project (HCP) (Van Essen et al., 2012; Van Essen et al., 2013) sample of 1,022 young adults"
^ it's a shoe size study from here out so of course they find something.