The whole argument relies on the fact that "being good at programming contests" is a factor that was considered on interviews (and even given too much weight in the decision). There is absolutely no hint of that at all, and having been on the hiring side in FAANGs I can safely say this is not the case.

There is no paradox here because the study is not done on a pool of candidates that are pre-filtered based on this specific parameter, only on what it correlates (being good at LeetCode, which makes you pass the interviews).

It is funny that in the second article you linked the author says "My point here is that you can tweak these variables and end up seeing correlations with pretty much any value.", because this is exactly what he does. He manipulates the problem until it turns into a Berkson's paradox.