GMC have been vague in a number of their announcements.
"substantially different terms" means either
1. substantially better terms; or
2. substantially worse terms.
Do you really think his counter offer was on better terms than the original offer.
Would the Board have considered this new offer and deemed it to be unacceptable, and have subsequently rejected the new offer if it was on better terms?
Stop clutching at straws.
I'm guessing you've answered no the questions above, so therefore it follows that the offer is on .......??