I don't really disagree with what you said about most people who are making that particular argument.

Fair enough.

Maybe I have gotten mixed up and it was Mackie who conceded to Plantinga? Unfortunately, I can't really check at the moment

Well, having looked into it, it appears that Plantinga wasn't a compatibilist, while Mackie was. Their respective arguments assume their favored version of free will. Wikipedia thinks that Plantinga's arguments are generally agreed to be valid if* you grant incompatibilism, which is a big if; the LW consensus seems to be compatibilist for obvious reasons. I can't find anything on either of them conceding, I'm afraid.