We’ve been migrating from version 2.5.16 which was released around 2009. Discourse-related part of the forums migration has been written based on @techAPJ’s version (my tutorial about our version has been already mentioned: Migrating from Jive Clearspace to Discourse), and it was also based on getting information from CSVs, but we had to provide support for attachments and some other features in additional to the original version.
The export from Jive (or Jive Clearspace as it was called around version 2.5) has been done via custom PHP scripts and relied on the REST API provided by Jive in a standard distribution: https://docs.jivesoftware.com/jive_clearspace_community/2.5.16/RestServicesReference.html, from what I see, they maintain their APIs in a good shape (they are enterprise solution after all which requires a lot of integrations), and there is an API for their current cloud version: Jive REST API v3.14, but version 5.0.4 has been most probably using the previous version of this API and it’s documented here: OpenClient REST API.
So I’d recommend you to develop an export via API which would export CSV files similar to those described, and then import all the content to Discourse.