Competent people do pass the filter, but what Leetcode interviews do filter out are people with a shred of creativity who just won't submit themselves to the drudgery of studying useless crap for an interview – the kind of people who need purpose to always align with work or part of them dies. Avoiding these people is great for a company that's all about boosting it's ad revenue, and also great for the applicant who dodged a bullet.