Filtering on anxiety levels doesn't mean you're selecting for confidence generally, it means you are selecting for people who are not anxious in the interview. If you practice for 1-4 hours per day for a month or more (as I did last time I was on the market) on a particular skill (like tech interviews), you become more confident at that thing. Grinding tech interview practice works, which is not great, for either side, because it's mostly orthogonal to actual job performance.

Impostor syndrome is most present in elite environments though. It's a syndrome because someone is genuinely talented but surrounded by high talent for each person it's easy to identify an area where they are better than you. In some cases it's easy to feel like you're not the best at anything in your work social circle.

Some people have high anxiety levels but still perform well under pressure. The anxiety gets sublimated into other behaviors, or bottled up until after the interview, or meditated away, etc. Even people that can handle high anxiety levels can still get impostor syndrome.