To be fair, this contest measures the performance of six people. The education that is most directly tied to IMO performance is specific "math for the top few students in the country" stuff which is usually some specialist math camp thing. So I don't think it necessarily means that much about a whole country's math education, if their IMO team doesn't do well.
Don't know about other countries. But in the USA, the six people who make the team are selected among high schoolers after rounds of competition. It does reflect very much a country's math education. But math education is just part of a much larger equation.