DOTM was not shot in 1080p. It was 35/65mm. 35mm can hold the same level of detail as 4k. Nothing theatrical (well, major films anyway) is shot 1080p unless its specifics for a particular camera for a particular shot.
ROTF was 35mm (which can be scanned to 2k) and on the Red (which shoots at 4k).
I would suggest that 2k masters are what was aimed for, as they were suitable for digital distribution and 1080p video transfers. I would assume you'd see some 4k remasters of bigger titles as the format moves forward. They likely went off existing 2k (FHD) masters for the previous films when editing this trailer rather than back to the original footage.