I think you may be misunderstanding the implication. It's not that he's a postdoc that's surprising, but that he's a statistician who happens to have solved a major problem in convex geometry. The following quote sums it up:

"Chen is not a convex geometer by training — instead, he is a statistician who became interested in the KLS conjecture because he wanted to get a handle on random sampling. “No one knows Yuansi Chen in our community,” Eldan said. “It’s pretty cool that you have this guy coming out of nowhere, solving one of [our] most important problems.”"

The great thing is, the reaction of the community working on these type of problems was to understand and verify the result. And once it was, all credit to him.