I think your point here is relevant.
One can never truely evaluate its own competence.
A degree, or good reviews from collegues are good indications you are competent. But also these are not proof: it could be a result of incompetent collegues, or an education that was not that good.
Not having a degree, but saying you know for sure to not have any Kruger raises lots of eyebrows for me: you do not know what you do not know.
Coming back to op's original question: the correlation comes from that education shows you what you do not know. You are getting involved with all kinds of subjects, and you get a grasp of how many there is left to learn and how smart certain things are. You might for example have never thought about the complexity of a compiler. This can make you feel dumber than if you would have never found out these fields existed.
Imo I think kruger is much more harmful than imposter
What's a subreddit?