The tutorial is pretty clear about where to put files. It uses Maven to build an AMP. It says nothing about putting a JAR in shared/lib.
If you are choosing not to use Maven to package your files up as an AMP which you then install in the WAR, you can put your files in the expanded alfresco WAR, even though this is not recommended. As long as your workflow XML is somewhere on the classpath you will be able to load the workflows into Activiti via either the workflow console or a Spring bean.