That is the level of math you need to do these problems with a little brief understanding of what certain concepts are. There is no calculus etc. The vast majority of IMO questions are applying the base rules to new problems.
There are entire fields of math with exceptional people trying to solve impossibly hard problems that utilize quite literally 0 calculus.
Many of them are also questions that eventually end up with proofs or solutions that only require very high level understanding of basic principles. But when I say very high I mean like impossibly high for the average person and ability to combine simple concepts to solve complex problems.
I'd wager the majority of Math graduates from universities would struggle to answer most IMO questions.
Okay, let's see you try any one of the past IMOs and show us your score.
It's really hard.
See my other comment. I was voted the best at math in my entire high school by my teachers, completed the first two years of college classes while still in high school. I've tried IMO problems for fun. I'm very happy if I get one right. I'd be infinitely satisfied to score a perfect on 3 out of 6 problems and that's nowhere near gold.
Olympiad questions don't require advanced concepts except maybe some classical geometry techniques that you wouldn't normally encounter in modern research mathematics. But they're fundamentally designed as puzzles. You need to spot the tricks.
This is a ridiculous understatement of the difficulty of getting gold at the IMO.