It's a specific hallucination from the book. I guess that image was the last straw for him, and it does mark a significant change from someone who had been previously driven to show that he can do everything on his own and not need anybody to this image of someone from the future whose very existence depends on his getting out from under that rock.
I also wouldn't call the style of the film cheating. I think it's part of what makes the film feel so unique, because we've never seen something like that, LIKE THAT. It wasn't just a bunch of style to try and distract you from how boring the movie really was. The style and the story marry to make that film. To wish for a different style would be to wish for a totally different film.
I guess I don't believe that one can talk about presenting a story with a different style and still be talking about the same film. I don't really like that kind of film criticism because I think it's a way for the reviewer to talk about some imaginary film instead of discussing the film as it is. It would be a different matter if the reviewer discussed more detailed elements of the film that could have been pushed more this way or that way in order for the film to be a more realized version of what it was trying to be, but broad criticisms of the fundamental aspects of the film become relatively meaningless, not necessarily because the reviewer is wrong, but because we can't really have a conversation about broad fundamentals, because we might have very different feelings that we associate with such broad ideas, and so we won't be talking about the same thing. So then it just becomes a really wordy way of saying "I liked it" or "I didn't like it," with no significant justification.
It's kind of like when people have an argument about religion--most of the time, absolutely nothing is being communicated between the people having the argument because they were talking about two completely different things in the first place without realizing it, and then they curse the other for being oblivious to what was so obvious to them, as though something just being obvious to you has any real validity in a debate/argument/analysis.
It's not easy to really evaluate a film and make intelligent and meaningful criticisms of it through clear arguments. (That's why I don't do it.) Really, all that we do here or anybody ever does on forums is say that they liked something or didn't like something. Their reasons for feeling the way they do aren't really illuminated by using more words or specifying which parts of the movie they liked, and adding why they felt that way tends not to really illuminate anything because it will probably be just another thing that they feel that you might not feel. Everybody's review of the film so far on here is just a variation of "I loved it/liked it/thought it was okay." That's pretty much what happens on movie forums, because it's rare that someone will say something genuinely insightful in a post that they spent 10 minutes on, and no one's gonna spend days crafting a hefty essay to put on a movie forum.
Oops. I kinda just rambled about a bunch of stuff at once there. I hope something in there made some sense to somebody. I'm tired.