Export synthesized top-module and import in another project

Things are different with Pro. See slide 11 of that training I linked you to. I/O are now exclusively top-level (periphery). With Standard edition incremental compilation, yes, you could get away with creating a .qxp file that included both core and periphery resources. However, it greatly complicated the integration in the final design. By putting a line in the sand, so to speak, between core and periphery resources, the flow works much better for a team-based design (less file transfer between team members; just the .qdb and optionally .sdc).