Well, it is the only LLVM language on the list. Even so, Nim transpiles to C and should be fairly comparable unless it is because of the async part or like you said they're cutting corners. I guess it's possible they're just that good, but it seems unlikely.
It does seem that immature languages tend to win a lot of benchmarks, likely because they're still cutting a lot of corners. Still, though.
I only wish they picked a more "Enterprise-friendly" name...