Okay, okay, people are probably gonna hate this, BUUUUT I have a weird theory (and it is NOT that Finn is in a coma crap)
AAAANYWAAAYS, we saw Finn as an old man in Puhoy and had me wondering... what if? What if Finn is actually an old, depressed man living in a post-apoctalyptic wasteland, with nothing but his pet dog, an old video-game system, the creepy older guy next door who lives with penguins, a small group of other survivors, and a collection of candy.
Maybe the radioactivity caused Finn to hallucinate that he was younger and living in some crazy magic place.
The Enchiridion might just be some old random book, and maybe after an incident where his tree house almost caught on fire he became a pyromaniac and fell in love with a ball of fire?
I know this theory is 100 % false and it's just plain stupid, but I got bored and started wondering about it.
Please don't freak out in the comments, I know it's stupid.