It is not free.
Only it should be mentioned clearly.
He's not, indeed, but that's why the GFDL was created. [1]
Interestingly, the GFDL is designed to be appealing to commercial publishers. [2]
If you advertise your project by stating you are convinced of the "power of Open Source", but don't make use of that "power", how much credible is that?
No matter if you find this argument convincing or not, it has nothing to do with the difference between Open Source and Free Software. It's just about adverticing a proprietary project with "power of Open Source" versus advertising it with, say, "power of Vim".
Many people believe [...] that you should not charge money for distributing copies of software, or that you should charge as little as possible — just enough to cover the cost. This is a misunderstanding. Actually, we encourage people who redistribute free software to charge as much as they wish or can.
In other words, it is important to distinguish paid software from proprietary software, otherwise the situation and arguments become confusing, indeed.
However, I'm sure the same people will happily spend $100 for a book that teaches some open-source tool. Or at least they would never complain that there's a price for it.
Why?
At the moment we seem stuck in the middle of this transition and you either pay for a locked down copy or you get the Free copy for free. But Kickstarter seems like it's getting mainstream so the concept of paying in advance for something that is built to the specifications of the user may at last become common in software.
It's a pity that the project's state is totally unclear (at least to me, a casual visitor). Is it meant to become a community project? Is it a demo of something that will be sold in the future? The explaining text behind the "Unlock Levels" button adds even more confusion:
> Additional levels are currently being developed for you to play and enjoy. The 3rd level will be available for FREE only for players who sign up for my email list. Sign up now! You'll be notified as soon as the level is up. The level won't be available for unregistered users.
On the one hand, it sounds as if it is free, with some "forcing" to make more users join the project mailing list. On the other hand, this is totally discouraging contributions (because you aren't allowed to see the work-in-progress version), so maybe it's meant to become for-sale in the future.
I'd love to see it developing either way (although I think the community way is the more appropriate for this project). But as of now, it seems to unify the disadvantages of both worlds: Advertising the mailing list in a way that appeals neither to people who are willing to pay (as there's no clear pricing plan), nor to people interested in contributing (as it seems to more about announcements/"newsletters" than about how to improve and to help).