It's not plausible, because if it were offered, it would have been accepted. How do I know this? Because the auction market for MTG art is, Underdeveloped" at best. It's not like someone turning down $2.5 mil for the Hulk 181 cover. You have a pretty good idea what that would go for based upon auction sales, etc. And, I dare say, that if the owner of the Hulk #181 cover was offered $2.5 million outside of an auction, they might well take it. That's 3 times the value of the Hulk 180 1st appearance panel page, and no item of OA has hit 7 figures in an auction setting, yet. (The rumored private 7 figure sales, notwithstanding).