I was surprised by that too -- is the growth mindset not that helpful for smart children?
I suspect that somehow the message "the other kids will catch up with you" got across, and become a self-fulfilling prophecy. Or the lessons were focused on the parts where the slower kids could catch up, ignoring the parts where the faster kids could get further ahead.
I don't think many researchers and educators are actually claiming that things like adopting the growth mindset will "close the gap" - that would be a spin added by journalists, etc.
I get somewhat when I read about "closing the gap" as a goal of education; it gives the impression that a policy that improves all children is strictly worse than a policy that improves only stupid children by the same amount.
Post by fellow LW reader Razib Khan, who many here probably know from the gnxp site or perhaps from his debate with Eliezer.