I went to MIT and knew a good number of people who were confident in technical interviews. The problems in interviews were much easier than what I got in class, or even CTY CS classes in middle school.

While there's definitely good people at non-elite universities, selection bias is at play here too.

For example, in the senior year case, you're just looking at people who were concerned enough about interviews to go to interviewing.io during their senior year. Most people I knew at MIT had a full time offer they were happy with from their Junior year internship by their senior year.

You can actually see the impact of this in your graphs too, notice that the % of people with a 1 or a 2 in elite schools increases for senior year vs junior year. People who have been doing well in technical interviews and have good return offers won't spend as much time preparing.