Did you read the link you posted? It says exactly what I said. That you can’t choose new content to sync without importing it first.
Ahhh… You are talking about adding new content to your library and not about synching content between your library and a device.
However, the requirement that you add content to your library before you can synch it to your devices is not a change from the way things worked in iTunes at all.
Here is a link to the correct information on how synch works in the Finder of Mac OS Catalina.
https://www.imore.com/how-sync-your-iphone-and-ipad-your-mac...