Contact/Lead Sync between Customer Insights Data and Journeys
I'm not entirely sure of your set up here. However, to be able to have leads/contacts integrated across CI-D and CI-J, you need to be using either the Power Query/dataflows Dataverse connector, or the native dataverse connector. You can see that here,
https://learn.microsoft.com/en-us/dynamics365/customer-insights/data/integrate-d365-apps, under requirements as the second bullet. This allows CI-D to backstamp/hydrate the related dataverse environment and mark the leads/accounts/contacts with their customerprofile ID which then links them to the other CI-D tables created in the related dataverse.
Once you have that connection established the other stuff should work. But I do see your recommend post which requires you to enable that CI connector in CI-J. I just went through this with MS support and they didn't have me do that, they had to turn something else on in my environment that allowed me to use CI-D tables in our CI-J real time journeys segment builder. I would presume it would also help with using that data for building journeys too, but haven't done that yet. Might be worth a ticket with MS to see if they can enable it for your environment.