If Weiner intended that Don made the Coke as, and I believe he did, then he should have done it slightly more explicitly....it was just kind of abrupt from Don at peace and then aCoke ad, from which we are to infer that the ad is part of the narrative....
Seems like he left it slightly vague as a cheap trick to get people talk about it and leave just enough wiggle room for other theories to be derived....
I don't think that there's any wiggle room at all.
To me, the loose end is: Did Don's experience in this final season lead him to happiness and fulfillment? Or do you look at it cynically and think that his final "progression" was just like all of his others: cyclical, to be used as fodder for selling products to the disconnected masses?
I hated the episode as I watched it but in thinking it through the next day, I really like how Weiner ended it.