Chapter 18’s episode description does tease a “reunion,” so maybe there’s going to be a reunion between David and his father.
Full disclosure, as there’s been no evidence of this event happening, but there could be a third exciting reason Legion got an additional episode.
Since Season 2 of Legion is focusing on the Shadow King’s quest to get his body back, a flashback episode to their battle would be appropriate.
If that were true, one would imagine FX would heavily promote Stewart’s appearance given the show’s low viewership, but sometimes surprises generate more buzz than advance notice.