Were you expecting that people with high C would or wouldn't skip questions?

Wouldn't; that was the claim of the linked paper.

Is it feasible to compare wrong sort of answer with C?

Not really, if it wasn't caught by the no-answer check or the NA check.

Is it possible that the test for C wasn't very good?

As I said, it came out as expected for LW as a whole, and it did correlate with income once the CS salaries were removed... Hard to know what ground-truth there could be to check the scores against.