This just shows that IQ tests are garbage. Top level math contests are a much better indicator of your intelligence. I don't know what tests you're talking about because USAMO and Putnam don't have internationals.
And what mathematicians are you talking about? Take time to look them up because I can think of no examples of great mathematicians being "bad at proofs". That would mean they were bad at logic, which would mean they were bad at math. Being more famous for a conjecture doesn't mean you were bad at proofs. To even produce those conjecture requires deep understanding of the math leading to it. I can almost guarantee that there are no great mathematicians who would zero Putnam. I don't even zero Putnam, and there are plenty of people way better than me who still have no chance of becoming a great mathematician.
Do you realize how brilliant a mathematician has to be for you to even ever know his name?