What you complain about is a false-negative error: an engineer with 15 years of experience can't pass these tests unless he prepares for 3 more days. That can sometimes happen, yes.

The initial stated problem was different: false-positives, bad engineers practicing for 6 weeks will pass the test. That I seriously doubt is a real thing. If they managed to learn that fast, they must be good. They could be junior-good -- that's perfectly fine, you need to hire juniors too. You don't judge seniority solely based on leetcode results.