If someone responds with “10” and fails to answer basic questions about that specific topic you know that the person is either lying or is overly confident.
I imagine the answer might be "we wouldn't waste time talking to them", but then, you just created an incentive for a person who would otherwise rate themselves as "5", "6", or "7" to pick a higher number in hope of squeezing through. You then penalize that in the interview process. Seems pretty counterproductive?
To your point, though, even saying 5 and not being able to answer basic questions would be a huge red flag.