They have exercised the cargo door option, but only on some of them initially. Budget issues and a need to field them quickly means two tranches will be delivered - the first will be the aircraft as is (boom, pods, etc), and the second to be delivered later will have the cargo door and datalinks. The early aircraft will later be retrofitted with all of the mods.
That explains a lot. So the MRTT's cargo door is not a "build to order" option, but a pre-engineered post-build modification option. That approach was a significant handicap in the Northrop Grumman KC-30/45 offer. Airbus still offers the -200F freighter, so I still don't get why they do not offer an MRTT based on the freighter rather than the passenger version. As France's order shows, there's definitely a market for it.