You are correct, he is wrong.
Remove your tail light and you will see there is a connector block with several terminals. If I remember correctly there is an empty slot that will power the license plate light. I think the license plate wire has a plug on the end of it and it will plug into the connector block.
I used a volt meter to make sure there was power to the terminal when the ignition switch was turned on.
I think the connector may be universal for both Electra-glides and Stree-glides.
Be careful not to connect it to a turn signal terminal.
Good luck with your project.
If you have a manual that has the wiring schematics, it may show the wiring for the connector block.