Ah, thanks for making me think more about it! :D
First of all, if writing was Tom's true dream, he ought to have done something concrete towards it. But let's not talk about that, because he probably has a lot of reasons why he can't do it.
Why Sam doesn't exist:
If writing is your dream and you have a day-job, you can find maybe 1 hour of worthwhile free time that you might spend doing something else. Say Tom doesn't tell anyone about his writing habits, and just does it alone (apparently Tom can achieve anything then), does he write enough?
When you start anything big, the excitement is very high, you might put in upto 6-7 hours of writing the first day, but you will always get to a point where that initial excitement is gone and this is the pivotal point for any project. You must either struggle through it and stick with your project or ditch it. This is when you struggle for motivation, struggle to find content, struggle to decide whether you should keep at it or just drop it.
If you made a public proclamation, your friends will keep asking about it, the closest ones for sure. It's embarassing to accept that you didn't go through with something that you were so passionate about, because accepting that you stopped to someone else is simply an indirect way of admitting it to yourself. You really wanted to do something, and you couldn't go through with it. What does that say about you? How did your perception of yourself change? Negatively, I argue.
Someone who doesn't tell any of his friends anything and just publishes a book doesn't exist. That just doesn't happen. Everyone slowly moves towards their goal, they don't just GET there.
Why this is a generalisation:
Everyone is different, following your dreams and passions is a topic that's been heavily written about offline and online. People have always wanted that single formula to get there, to become a writer or a film-maker or whatever. The fact that it's been so long, and there's no formula yet is enough to prove that no such formula exists.
No such formula exists because EVERY SINGLE SITUATION IS DIFFERENT. That's the core reason for my dislike of this answer. This is an overly simplified answer about a very complicated, specific problem: How do you get to where you want to be, from where you are? There's no one single way to do that, and consequently, there's no single way you can completely screw it up either. Every step in any direction is a step towards the goal or away from it, but making progress is more important than making sure that each step you take goes towards the goal. (Some of Casey Neistat's videos have really detailed explanations of goals and life and the path to take to get there. All of the above has formed from my own experiences with ambitious projects and accomplished people's views on the topic)
Nit picks:
The question asks for psychological facts, facts are generally supported by research that is either biological or done using control groups and analysis. They are not things that you can come up with using fictional characters and thought experiments.
The comments of that answer have some references to a 2009 paper. That paper is shaky, IMHO. If you believe that the experiment that was performed was to the point and is an accurate representation, good for you. :)
Relatability:
There's no getting around the fact that more than 10,000 people thought that answer was a good one. Maybe the answer is relatable, that doesn't make the answer true. Just because something happened to 10,000 people doesn't mean that's the only way to do that thing. There are other ways too. :)