I read that as "Nick in New Girl" and it made just as much sense.
That article never states he was offered the role. My guess is Fincher was taking meetings with a handful of actors who were interested/who he liked for it and he wanted to meet Hamm and Weiner/the Mad Men folks said no, likely due to shooting schedule and nothing else. AMC certainly wouldn't have put the kibosh on their leading man starring in a gigantic movie that would only serve as great press for their final season. I can actually see this as some sort of play by his reps - rumor gets out he wanted the role, rather than say he ultimately was passed over in favor of Affleck (who I think was much better casting, honestly), they blame Weiner. Just my educated speculation, no real intel here.