The sync is controlled from the finder, but the available content is controlled from the TV app.

I'm not certain where you got this impression, but it is incorrect.

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...

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.

I know it’s not a change. That’s my complaint. I would expect to be able to drag files from my hard drive to the device. The fact that I can’t makes the experience worse than iTunes.