Is pub.flow:debugLog service synchronous or asynchronous in webMethods 8?
I do not believe that the services in pub.flow folder are async. They would have mentioned it in the documentation if this was the case (http://techcommunity.softwareag.com/ecosystem/documentation/webmethods/wmsuites/wmsuite8-2_sp2/Integration_Server/8-2-SP2_Integration_Server_Built-In_Services_Reference.pdf).
One simple way to confirm this would be to stop your broker and see if the debug service still works as expected.
You can create an async logging service using the Broker, but you can also create an async logging service within IS. A typical example is log4j implementation within IS.