Syncing iPhone for first time to a new computer

I'm setting up a new Windows 11 PC and I want to do the first sync via iTunes of my iPhone that was previously synced on my prior Windows 10 system.

When I select "Sync Music", a pop-up message seems to say that music on my phone from "other iTunes libraries" will first be erased before the sync is done against the iTunes music library on my new PC. But that library is empty, since this is a new computer. Am I misunderstanding the message? I would expect the first sync to upload all music on my phone to the new iTunes music library, thereby populating it. Am I correct, or will I wind up with all the music on my iPhone deleted?