The Javadoc currently reads as follows:

Interface to be implemented by @Configuration classes annotated with @EnableAsync that wish to customize the Executor instance used when processing async method invocations or the AsyncUncaughtExceptionHandler instance used to process exception thrown from async method with void return type.

However, any bean implementing this interface would do. And having more than one bean of that type would fail. I don't remember if the implementation looked specifically for @EnableAsync but that's no longer the case.