I gave this the exact same score as I did for the main drama, so please don't mind much.
Clearly, this IS the 21st episode of Romantic Teacher, Doctor Kim. This is not MDL's fault, however. The production team decided to end Romantic Doctor, Teacher Kim at 20 episodes, and label this one an Extra Episode instead. With that being said, you cannot watch this as a standalone drama special. You will need to watch Romantic Doctor, Teacher Kim first before proceeding to this one.
I wish they didn't label this an extra though. This episode is what makes the entire series actually complete. It does not start introducing any of the characters again as a new production. It doesn't jump ahead or back in time. It continues smoothly from episode 20 and answers a lot of questions left hanging from the main series. It has a lot going on as well. It's not just a dragging 'filler' kind of episode. So many 'romantic' things are going on here :)
This is definitely a must-watch. Consider your progress 20/21 if you haven't seen this yet.