jacamera is on point. Perhaps it's not "leetcode" but it's a seemingly one-dimensional, time-constrained quiz on a specific skill, parsing messy JSON (your words), with you as the sole judge. Personally from experience, when I approach a new API, I recognize it's likely to take a few iterations, based on how that API's data integrates with the rest of my program, to determine the best way to deserialize the API's responses into objects. If I've only got 45 minutes, yea, I'm just going to map it quick and dirty and it's going to look ugly.
Your observation about F# may be valid, but this does appear to be a test of one specific use case for a language, not how productive people are when building entire applications with them.