Earlier quoted context omitted.
Strange that nowhere on this landing page does Mindustry advertise itself as "free software". In fact, there is a very prominent link to buy it on steam for $9.99. A bit off-putting. As developers of course we associate GitHub with FOSS, but would a layman? I guess the thinking is anyone who doesn't know to visit the repo and `git clone` probably requires the steam installation? $9.99 however is not cheap many places…
I've seen this pattern before and think it is very interesting. It has the benefit that the people able to contribute get the game for free and the people who cannot contribute have to pay. On the other hand, it is very programmer-elitist.
If you don't want to pay now(for whatever reason) but want to play it. You just do. (And I highly suggest you do it if you haven't)