Responses
I think it's all about interpretation...we don't even know how those who have ostracized or bullied Amber have reacted after her death. Most likely play it up for drama just for five minutes of attention. But with Leo/Lucas...he might have taken her death the hardest.
So perhaps, in a way, it's the universe's way of giving them the ending that they deserve.
1. MC really did transmigrate and Lucas's completed manuscript doesn't have all that much to do with the story
2. The whole story is from Lucas's completed manuscript - i.e, he replaced her self insert with what he knew of her personality, and wrapped up the story.
The first option would be a good way to wrap up the story of those MC left behind.
The second option reads like wish fulfillment where he turns his supposed stand-in into the real love interest. Unless MC specifically wrote notes about Leo being a Lucas clone in her manuscript somewhere, I can't be certain if he was projecting or not. Also, if this is the case, MC's initial monologue about her feeling jealous about Lucas's novels and going doki-doki in his presence reads very different.
I hope author had option 1 in mind.