Why would ESPN negotiate with the ACC for the right to stream Clemson games when they already own those rights? The real issue comes when the SEC says Clemson TV rights belong to the SEC. That's when ESPN bows out of the ACC contract. Without the eight most attractive teams the cost is not worth the revenue.
The issue with that is product under contract with ESPN would have changed and given ESPN the legal right to exit the agreement. Without the TV revenue, the conference dissolves. I will be surprised if it happens in the next 5-10 years.
THAT is the point of the GOR. IF Clemson leaves the ACC to join the SEC, the SEC will not own the rights to the Clemson games. You are correct that ESPN already has a contract to broadcast the ACC games. However, you are incorrect in thinking that the contract with ESPN would have changed, and that ESPN could just pull out of the contract with the ACC. The Clemson games would still belong to the ACC. The ACC would still get the revenue from the Clemson games. ESPN could still broadcast those games under the current ESPN/ACC contract.
The big problem with teams leaving the ACC is that they do not bring any broadcast revenue opportunities to another conference. My post that you originally responded to was saying that the GOR contract provides that any current teams in the ACC still own the broadcast rights of any teams who leave the conference. (at least the previous GOR which is publicly available) I have not seen anything other than wild speculation that 8 teams leaving the conference would invalidate the GOR. (In fact the GOR specifically says the it can only be modified if EVERY current member votes to modify it.)
One thing to think about is how Texas exited the Big12. Texas has more money than any other NCAA sports program. Their yearly sports revenue is around $240 million. They could easily forgo two years' worth of broadcast revenue without it affecting their budget significantly. If ANY program has the ability to challenge a GOR, it is Texas. If they challenged and lost, it would have been painful, but more like a bruise than broken bones. They decided to wait instead of challenging it.
EDIT: I just looked up the 2022 numbers and Ohio State had more revenue than Texas. However, the next team below Texas was $24 million below Texas in revenue. Ohio State - $251 million, Texas $239 million, Alabama $214 million. Of that $239 million, only $42 million was from broadcast rights. If they had forgone the broadcast revenue, they would still have had more revenue than all but 5 NCAA athletic departments. The ACC school with the most revenue was Virginia at $161 million. $42 million of that was broadcast rights. That would hurt Virginia a lot more than Texas. (What I called "broadcast rights" is actually media and NCAA distributions in that report)