All DC-9 and Super 80 DC-9 (MD-80) series had a jettison-able tail cone emergency exit with escape slide.
The door to this from inside the passenger cabin was the same door to use the ventral passenger stairs, but in an emergency you used a ramp/walkway over the top of those to gain access to the tail exit and slide once the tail cone was popped and dropped.
In the Baby DC-9's without rear ventral stairs this aft cabin door was for emergency use only and the aft cabin crew seats were attached to it, once the door opened there was a ramp to access the exit and slide.
All DC-9 and Super 80 series had a rear ventral drop down passenger stairs, except most of the small baby DC9-14/15 series, although those built as an -15RC (rapid-change pax to cargo and combi) some of these had rear ventral stairs fitted, and all of these RC's had 2 more over wing exits (Thus 4 like DC9-30)
Rear aft cabin service entry doors were only fitted on the Super 80 series.
The Super 80 in the video doesn't seem to have ventral stairs so maybe have been the prototype? However all production aircraft did.