Right.
He’s a Rule 5 pick until he completes a year on the 26 or until someone buys (or trades) for him with Boston.
Song is like a leased car, and the Sox are his auto dealer. If Philly wants out of the lease in less than the one year (re: agreed upon lease length), they can exercise their “option to buy” him from Boston and work something out a deal. But until they do, the terms of the lease are in effect, no matter who’s driving.