IMO questions are to math as leetcode questions are to software engineering. Not necessarily easier or harder but they test ability on different axes. There’s definitely some overlap with undergrad level proof style questions but I disagree that being a working mathematician would necessarily mean you can solve these type of questions quickly. I did a PhD in pure math (and undergrad obv) and I know I’d have to spend time revising and then practicing to even begin answering most IMO questions.