That's not what the person said. Give a little bit of the benefit of the doubt when interpreting posts. Using the context of a person who grew up and was educated in the Anglosphere. Obviously the ivy league is going to be one of the more attractive options for finding a larger group of elite mathematical researchers. They have a ton of funding compared to most places and draw in many other brilliant people from around the world. That doesn't mean there's no elite thinkers anywhere else, just that it's inevitably going to be a strong contender for where a very bright person looking for that kind of environment would consider.