I was just entertaining OP, I don't think IQ correlates that well to SW engineering job performance. It's only one of many many factors (albeit probably the easiest thing to test for - testing curiousity, emotional intelligence, motivation, work ethic and resilience is much much harder).

But if we're saying Leetcode is simply testing for IQ and not really for software engineering ability, why not just test for IQ? IQ tests are designed in a way that after a certain threshold it's pretty hard to improve in them - they actually test some innate ability. Leetcode tests how well you are prepared for Leetcode, and perhaps how well you do under stress. It correlates only slightly with intelligence and even less slightly with programming ability.

I think the fact that leetcode needs to be studied incorporates work ethic/motivation/resilience alongside IQ. Also, at some stage in the interview process you have to interact with people which will expose your social skills/emotional intelligence to some degree.