Reputation in the field is not a proxy for general intelligence. That there are only 10 people with the reputation to make it worth them assessing the proof does not mean those are the only 10 people with the smarts to understand it.
And yes, a large part of being able to work in a specialized field like this really is about having the specialized knowledge and vocabulary, rather than general intelligence. If this is anything like most proofs in mathematics, most reasonably intelligent people would be able to understand it given ten years of studying the field. It's not about being smart enough, there's just so much existing knowledge that you need a lot of specialization to be able to really contribute to any given area.