Yes, what you're describing isn't Work. We may have been conditioned to use the same word for both things, as you did, to make it appear they are the same thing, but they're very different.
Eh, you have to work for yourself too. It comes down to who are you working for... who's "will" are you sustaining? This is about willpower and using it for your own ends instead of others whether that is work or some other activity.
I know this is Hacker News, and so someone "working on something" on their own is typically interpreted to mean "building a startup", which I would still call "work" in the more classic meaning of employment, but "work" doesn't have to mean that.
A friend of mine retired early and spends a lot of time in his workshop building fun things for himself. That is work, but it's not "work" in the way people think of the term.
One project I want to do some time is design and 3D print a model roller coaster, including building a train to travel over it. That's work, but it's not employment.