The BBC article notes that this section of fuselage can optionally include an extra emergency exit, but that Alaska does not select that option. Still, it may be constructed with a false opening the same size as the exit door which could have been there.
It seems odd to me that an emergency exit can be optional. Like I get that airlines reconfigure seating, number of bathrooms, storage, etc, but emergency exits seem like there should be a fixed number based on maximum distances between all exits or something.
Another comment elsewhere says it's not really optional, it's just down to the number of seats the plane has. For certain seat configurations it's required.
