2015 is a long time ago in tech, especially when it appears that IBM did not sell of their chip R&D labs. to Global Foundries.
I think it will all depend on the wording of the contract of sale. If it is the 2nm gate-all-around technology, then chances are that it did not exist in 2015, so it will probably revolve around what was detailed in the contract for derivative technologies.
There may also be something in any time-based exclusivity, which has probably expired now. In addition, as far as I remember, IBM claimed that GF were in breach of contract over late delivery of Power9+, or was it Power10, and the associated z technologies, which caused IBM to look around for other foundries to make their chips. In order for other foundries to build these chips using the designs, they would have to be given details of the processes used to create them.
In my view, this probably means that there were clauses in the contract that allowed IBM to move manufacturing to someone other than GF, especially if there was a breach of contract. IBM has enough good lawyers to not have some get-out clause once exclusivity had passed or the contract was breached.