My inherent slothfulness does not grasp your mortal sense of time. Oh all right...
Actually I'd seen some contrasting theories that made me second-think it a little but I still reckon I'm right.
So the part of Ruka's doujin that obviously goes awry is the ending. It's BS. Not only is it unsatisfying, but it's completely unconnected to the plot of the story up to that point. The character goes from trying to prove herself as a writer to "screw it, Got a man, I'm good!" in less then a page.
There's a lot of obvious parallels with Ruka's own situation here. To save myself a lot of typing, she is, in fact, trying to skip straight to her own happy ending and isn't afraid to force the issue. She doesn't just want Hayate, she
needs him, as she just told Nagi like two chapters ago. She went from writing a bucket list with "boyfriend" at the top to noticing Hayate was pretty hawt to hitting on him to, well, escalating every time she got turned down.
She worked herself up to a marriage promise within a matter of days, in no small part because she was increasingly frustrated with his obvious lack of interest. It's a one sided relationship and Ruka isn't oblivious enough to not see it, she's just lying to herself.
So that's the big lie, I think, but it's still a bit more nuanced then all that. Like the writer in her story, there's no clear path from point A to point B. The reincarnated writer is mad at these new-fangled light novels and determined to prove herself, but why? How? What does she actually want to do with her new life, and what kind of books is she going to write?
Basically, Ruka needs to deal with her own problems and decide what really makes her happy before she's ready for a relationship. She still does have the debt dragging her down, and that will have to be removed somehow before she's really free to do what she wants.
I've seen some people suggest that the big lie is that Ruka really really loves singing more then manga but ehhhhh I don't see it. Certainly she doesn't
hate singing other than that big black ball of bitterness she's currently directing at her parents but there's been no real hints or build up in that direction in the manga, and her doujin doesn't seem to lean that way either. Ruka is explicitly presented at being talented enough in both paths that she could make a career at either if she chose to do so.
In the end, she probably will give up on Hayate, at least in the "gotta make him marry me" sense and go back to being an idol but I don't think it will be as simple as losing a contest or suddenly realizing she likes music more then manga. She'll have to deal with her inner turmoil one way or another and then she'll be able to make clear decisions.
...Or she'll fall into such depression that she'll end up using a stone and fleeing to the RG but we'll just consider that a highly probable detour in getting her life sorted out.
On an different note, I really like the reveal that Kayura is secretly Ruka's biggest fan from way before it was cool. It explains a number of things in both the manga and the anime and makes their future interactions more interesting.