Quote from the linked forum thread:

MapReduceTask#executeAsynchronously() doesn't actually do anything asynchronously. It just returns a MapReduceTaskFuture containing a Callable that calls execute(). The only place I see that Callable being called is in MapReduceTaskFuture#get().

In other words, the task isn't actually started until you call Future#get(), which isn't asynchronous at all! On top of that, MapReduceTaskFuture extends AbstractInProcessFuture, which "implements" the get(long, TimeUnit) method by just calling get(). Which means that MapReduceTaskFuture doesn't respect the timeout parameters and will just run until it completes.