That doesn’t fit. You don’t discover you’re terminally ill and then brainstorm a game while you're out trying to find yourself. If you think you're terminally ill then your notebook is likely to be full of meditations on the day's endeavors and thoughts about your legacy, and your mortality, maybe even your family. You write about a game when you're nowhere near a computer, it's because you're imagining eventually being back at a computer. Or if you have the hubris to think someone will type your ideas into a computer and this game is gonna be your enduring legacy, then you write with an audience and try to tell people what they are reading -- not a private notebook.
But I think you're onto something. Non-terminally ill could be a different story.
This is almost certainly not what happened, being hyperspecific, but let me tell you a compelling possibility. If you were a programmer with a really mild form of epilepsy that was slowly getting worse and worse, you could imagine thinking that it was related to all of your time playing games and staring at screens, like you find that your eyes just kind of lock on a screen where action is happening and your heart starts beating too fast as your brain goes into overdrive processing too much action and you know you have to tear your eyes away but your eyes are not cooperating. Really uncomfortable and after a really bad episode you will probably feel like your brain is out-of-joint and any encounter with the wrong stimulus might send you back into an "acid flashback" type of situation. Possibly that sort of situation includes hiking for too long or so. Supposing you were of that mindset you might be thinking that this was environmental and, “I am going to get out in nature and these things will quiet down and I will plan my follow-up game." Mild recurrences on the trail are explained as just "no I haven't finished healing yet." Then triggered by heat and being lost and stressed in a Florida swamp, maybe you have another really big one. Zapping in and out of consciousness, maybe you don't have the mental wherewithal to figure out where you are and how to get to safety. Something like that.
Again, that particular scenario is deeply unlikely, but it shows off the basic features: he's writing in his notebook because he expects to return to work, he's taking an extended vacation in nature because he cannot stand to keep working in his current situation. Maybe.
I am really reluctant to post this, something about joining the internet's quest to solve this guy's case seems... I dunno. Fetishistic. I don't really have much to contribute except that I wish the scans were better so I could read this guy's notebook more clearly.