The article goes over that.
specifically how it wasn't the grade that was the issue, it was the speed of the course material. so once your son catches up, the problems will resurface because of the slow people. just now compounded by the social isolation and lack of physical development in comparison to peers.
Recess isn't socially isolated, class instruction time isn't particularly social, and a variety of classes are not IQ based.
One needs to select a learning environment that can provide skill developing challenge in both IQ-appropriate and socially-appropriate milieus. Note I didn't say "age appropriate", as that's not a real thing, it's a responsibility-abdicating proxy.