I've been using most of my free time in the last decade to write side projects (for my own use, not published ones), and I strongly agree with the author that small steps are key.
Although, reading your comment, I suspect that as often, generalizing this idea is probably an error, and I don't doubt what you describe works best for you and the people you mention from the HN thread (and thus, probably many others).
For me, product design (both professionally and for side projects) is all about dividing features. Any time I think about a feature, it doesn't take a long thinking time to realize there are several features in there. I keep dividing until I can't anymore, and then I prioritize them in a todo list, parts of a "feature" often ending up after parts of an other feature, because it turns out they're not that important.
This helped me a lot for side projects, because I've reached a point where the unit of time for writing a feature is the hour (only on side projects, the unit of time is the day, professionally, because there always are more complicated needs).
And this is what does it for me for side projects : knowing that when I start something, I'll have something useful in return at the end of my session, and having a clearly organized global todo list of all tasks for all my side projects. When I open my todo list each day, I'm generally excited thinking of what the topmost item will allow me to do, and it's only a few hours away!