It's under the GPL 3 license, so it's free software. Click the GitHub link on top for the source code. It's also free on itch.io. There's a link to that below the Steam link.
Even as a developer, if Steam were to recommend this game to me, I have no way of knowing it's FOSS. Very interesting strategy. I'll give them that.
I have enough misgivings about it though that I probably won't be playing it. Feels scammy. Like they are trying to essentially release a paid game, but with the support and goodwill of the FOSS community at their disposal.
I certainly hope all of the contributing developers are getting a cut...
Would you call it a steal if the man maintaining it basically work full time on it and live on it? I think play it for free feels more like a steal instead.
(And technically, this is one man project that accepts contribution, most work are done by himself, includes game mechanism and maps)