If you have not yet seen the episode this will be one big spoiler so...
Yes, unfortunately what many of us had suspected was true. The villain Gellar was a figment of Colin Hank's imagination a la Fight Club or, better yet, a la Psycho. The episode was followed by a commentary about how clever and unforeseeable the twist was to the cast... and that Colin Hank's didn't even realize that he's basically been playing Norman Bates.
The problem isn't that the split personality angle wasn't a clever twist, but that it took so many episodes to reveal it, allowing the audience to catch on. With so many scenes depicting Gellar and Travis talking, no one else interacting with Gellar, and Gellar popping up menacingly at far too convenient times... it was inevitable that people would catch on that he is a boogeyman.
Given how far into the season the reveal occurred, the writers would have needed to do something far more surprising... such as reveal that the real Gellar is alive but has brainwashed Travis into believing that he is always just behind him. If Dexter had eliminated Travis and thought the whole thing was over, only to find in a last moment shot that a real-life Gellar is standing behind him in his bathroom mirror... that, would have justified 8 episodes.