So yeah, this is nothing but a cash grab by AMC.
The article also quotes Weiner as saying:
We plan to take advantage of this chance to have a more elaborate story told in two parts, which can resonate a little bit longer in the minds of our audience.
There's economic and scheduling reasons to film everything at once. That, in and of itself, doesn't really speak to the motives of AMC (which strikes me less as an effort to make more money, than an effort to heighten the network's profile by keeping new episodes of Mad Men
in the air for another two years rather than one).