Fox does have O&Os in a number of smaller NFC markets and doesn't seem to have acquired stations in a non-NFC market in years while actively trying to get rid of stations in smaller AFC markets. The WJZY-WMYT acquisition in Charlotte comes to mind. On the flip side, CBS doesn't own and doesn't seem to have an interest in owning KHOU, the CBS affiliate in the fourth-largest AFC market and a top-ten market overall.
With this transaction, here are the Fox stations Fox will own in non-NFC markets: KTTV Los Angeles, WOFL Orlando/WOGX Gainesville, WHBQ Memphis, KTBC Austin, KRIV Houston. Only KRIV is in an AFC market, and even there the ability to beam in the Cowboys helps make up for it (Boston had nothing comparable). CBS owns WCCO Minneapolis, a market substantially smaller than Houston at a level Fox would have jettisoned long ago if it were an AFC market, and WWJ Detroit, a shitty station in a market smaller than Boston.