One thing that I think the author fails to accurately reflect is how fierce the competitive process to get in to TJ is. And moreover, the culture within middle and elementary schools which this competition breeds. When he states:
"Score high enough on an entrance exam—something hard but totally within your control—and you could attend a school"
This is frankly a far cry from what I actually observed. There was an exam to qualify; however, so many extremely smart and tenacious people attempt to do well on this test, that to outperform all/most of them would make you a prodigy among prodigies. It was not uncommon for parents to pay for tutoring specifically oriented around this exam. I feel like saying that this is "totally within your control", is not literally false, but grossly misleading to the actual situation.
I can't stress enough that the competition to get in to TJ was like the competition to get in to Ivy league colleges. In fact in many ways, the competition around getting in to TJ was not just analogous to the Ivy league competition, but a direct extension of it, as going to TJ would make your college applications more competitive. And naturally this extends back even before TJ, going to placements tests in the 2nd grade to qualify for GT elementary schools. I feel (and can attest with my own experience) that kids at this age have no conceptualization of the long term consequences of these kinds of gateway moments. More often than not, success at these stages is more a reflection of parental coaching (or lack thereof) and familiarity in navigating their children through these gateways.