A paradigm, in the context of APEX and APRS, describes the standards around specifying paths, and how digipeaters handle a path. In the older APRS protocol this was limited to the selection of path aliases and how they were handled. However with APEX the content of beacon, packet, and status messages also contribute as they have a indirect role in how packets are routed, specifically in how routes may be automatically discovered by an APEX client.
The APEX paradigm can be applied to some older host mode TNC that might not be otherwise compatible with the full APEX stack. So it can act as an intermediate step towards full adoption. However only a full APEX client will reap the full benefits of the APEX protocol.