Nobody wrote that it was about luck, they wrote that it was about the perception of the students that it was luck. So long as kids who they perceive as being as good as themselves are being rejected, they will see that as luck. Besides, there is an element of luck among all “rungs”, because that’s how holistic admissions works. Many kids with SATs and GPAs that match the top 25% of Stanford are rejected by Stanford.
People with impostor syndrome generally think that they achieved what they did either through luck or by subterfuge, and that their luck is going to run out or their “scam” is going to be revealed. a kid in the “upper rungs” doesn’t think that their 4.0 indicates that they are smart, they think that either their high school was easy, or that you don’t really need to be smart in order to get As in high school, and that they somehow managed to “fool” their teachers, but now that they’re in college, where everybody else is “really” smart, they will not be able to fool their professors and they will be shown to be a fake.
But there are many more kids at selective colleges in the “lower rungs”, and they need many fewer mental gymnastics to convince themselves that they do not belong in the selective school that they attend.
PS. I’m skeptical of the claim that social media is to blame for increases in impostor syndrome. First, because there is no evidence that it is increasing, only that people are more willing to admit to it. Second, before social media people only saw perfect families on TV and in movies, and what they saw and heard from their friends, family, and neighbors was just as tidied up and perfectly presented as the lives presented by anybody on FB or Instagram.