I know that in On Going TV Shows 'Special Guest' or 'Guest Star' is a Loophole that allows those actors to film Pilots and audition for otherwise schedule conflicting Roles (Think Locklear on Melrose Place in the 90s)
Not sure what it means for a one off TV movie...
as noted by the other poster, Ms. Jones having been 'retired' for several years and coming back for the reunion, it could easily have been an added 'thank you', as I doubt the project would have been approved (for better or worse) without Morticia, or a re-cast, or dare I say 'Fake Shemping'
-- her apparent Dual Role, as implied by the Credits here may have been a factor
then again it could simply be to call attention to her name, like the More Common "with" "and" or "____ as ____"; where the last actor listed is set apart from the others