I did a test, and it was as you suggested, Andrea. Starting with a copyrighted paper and doing a save as under a new name retains the metadata on the original paper file--copyright information, kit name, etc., even on a flattened jpg.
I tried something else, and started with a new empty file and used the place in Photoshop command from Bridge for one of the copyrighted elements in one of Andrea's kits, then flattened and saved the file. That file did not retain any metadata from the placed file.
So it's good practice to always start with a clean new file and drag over the papers and elements. You can delete the extra blank layer from the original file at any time. That way, there's also no chance you'll accidentally overwrite the original paper file as you might if you'd started with that.