It looks like the compiled binary just gets exec'd by the Lambda function and listens on a TCP port (specified by the environment variable _LAMBDA_SERVER_PORT) which accepts RPC calls to execute the handler using net/rpc.
The code: https://github.com/aws/aws-lambda-go/blob/master/lambda/entr...
Given that you have to compile the binary before pushing it up, it seems weird to announce "golang support" and not "support for any binary that talks $insert_serialization_scheme" over stdin/stdout or a network socket or some such, unless they're doing fancier magic.