If you're hiring software developers and you care about IQ, you don't need to test it implicitly; you can safely test for it explicitly, and there are several large, deep-pocketed plaintiffs lawyer targets who routinely do so. The idea that general cognitive testing is verboten in US employment is almost entirely an Internet myth.

People use Leetcode because they believe it tests for programming aptitude.

Nope, you'll get a lawsuit for discrimination if you explicitly test for IQ. That's why they do it implicitly. The scheme is very simple: hire people with the highest IQ, and since they have high IQ, they will figure at least something out.

It tests for multiple things, at its best: A basic work ethic to understand fundamental CS concepts. Sure maybe plenty of people can't write a binary search in two minutes unless they practice live coding a bit, but plenty of people do study, so it self selects for that type.

There are also people who, no matter what, could not live code simple tree traversals or bin search or something, and it filters on that.

Finally, there's a pattern matching aspect to it. Some of the best interview questions I got involved very simple algorithms, but it was obfuscated by the problem. So the 'trick' was to just think through the problem and ask questions. Not to have memorized something obscure.