From Flight:
Provision for the new exits – which boost the exit limit capacity from 189 to 215 passengers – “will be structurally installed as standard in all -900s, and will allow operators to decide if the door should be activated or deactivated”, says 737 chief project engineer Mike Delaney.
The deactivation feature will enhance the aircraft’s remarketability, says Delaney, adding that, when not used, the mid-exit area will be covered with a standard sidewall and overhead bin interior.
That sounds to me like it's a genuine door, only deactivated (and which can be reactivated), rather than a dummy one.