http://www.vanityfair.com/hollywood/2015/05/jon-hamm-mad-men-ending-explained?mbid=social_twitter
Straight from the horse's mouth: " It's a little bit
ambiguous."
The fact that Weiner didn't actually
show Don go back to McCann and pitch the idea, by its very nature, means it's open for interpretation. Yes, OBVIOUSLY Don came up with the ad. Yes, there were ABSOLUTELY clues throughout alluding to that fact. Weiner is DEFINITELY saying Don created the ad, no doubt. But he's doing in such a way to ALSO say, "BUT, you can read it another way if you want to." He's leaving that door open, ever so slightly. The very fact that we're even having this conversation means it wasn't defined as fact. And that was done on purpose. Hence the ending being "ambiguous."
Also, I LOVE Hamm's interpretation. That's exactly what I was hoping for / how I read it; that Don can exist as both a changed man for the better AND the creator of the ad.