It's not ambiguous. Weiner has already explained exactly what happened.

Don did create the Coke ad, as has been confirmed by Weiner himself.

T'was a joke, mate

I can see that — and it was my initial reaction — but after rewatching, it's certainly not as open-ended as the Sopranos ending, in my opinion. (Looking forward to Weiner revealing that Peggy obviously created the Coke ad.)

I don't think there should be a debate. I think that it's pretty unambiguous that Don created the Coke ad, and there is no shortage of clues that lead us to that conclusion. I kinda sorta see the Sopranos comparison, but not really.

The book is mediocre