It needs to be usable depending on how many passengers the interior is configured for.
So it has all of the door bits there. Maybe some parts like the emergency escape slide are not installed.
e: I should be clear that it's not usable as an emergency exit, as configured by Alaska. However the operator could choose to activate it later and install a usable exit.
If you are correct, then the implication is that the concern extends beyond door plugs for MAX-9 737s to all emergency exit doors on all models of aircraft sharing this design. This is somewhat reminiscent of the huge problem with the 688 (Los Angeles) class submarines, where the discovery of a faulty weld that had passed inspections raised doubts about all welds.
If there was a reconfiguration to a seating standard that required the extra exit, the plug would be removed and a proper door would be installed, with the associated interior pieces.
This is true.
However there's still common hardware in there to allow the plug to be installed and maintained. This is why it's a complicated set of kit vs just bolting in a permanent fixture.
Alaskan airlines chose a 178 passenger configuration for their 737-9, and so are not required to have a mid-cabin exit door.
Lion Air's chosen to go with a 221 passenger configuration, and so are required to have an operating door.
Obviously changing up the number of seats isn't done on-demand, you'd need to go for a refit/maintenance cycles.
But if Alaskan decided to change density, or sold the aircraft to someone else who decided to change density - then they could go and do this.
[1] https://youtu.be/nw4eQGAmXQ0?t=305 "The Boeing 737 Technical Channel"