Request handlers expect a synchronous ProgressEvent response instead of async

The invokeHandler() method in AbstractWrapper.java and handleRequest() method in LambdaWrapper.java seems to always return a synchronous response in form of ProgressEvent. With server side programming semantics, I would expect them to return an async response (like a JAVA future).

Is this a limitation of aws lambda or the cloudformation infra should use the async library for aws lambda? 