I said arms and back (I had to double check I didn't misspeak). The arms stick up higher than the seat. So the concave surface defined by the back and the arms is most of what will limit you getting through the straight edges of a doorway.

There are a few couches where the seat sticking out past the front of the arms causes problems, and a few handrails in stairways that you can sneak between the arms of course, but probably nine times out of ten it's just the obtuse angle between the back and the arms that is your constraint. And that angle plus the rest of the couch around it looks a lot more like the cutout in the linked article than an L. Which is kinda the reason I brought it up.