The problem: the game only downloads 0 bytes on a Linux-based OS, even when with Proton enabled for all games in the Steam settings.

The fix: Click/select the cog icon for the game in your library, then go to
"Properties" -> "Compatibility"
and check "Force the use of a specific Steam Play compatibility tool." Then select the version of Proton you want to use from the drop down. (If you're not sure which to choose, I recommend trying the one with the highest version number that is NOT a beta.)

Why this works: I'm not exactly sure, and have no experience with releasing games on Steam. Based on some quick research, however, my guess is that this is caused by a developer mistake. According to SteamDB, the game has an public, empty Steam depot marked as the Linux version of the game. This means that Steam tries to download a Linux version by default, but since the source is empty, 0 bytes are downloaded. Forcing Steam to use Proton, and thus download the Windows version of the game, fixes this.

...

I assume that this fix will no longer be needed either once this depot is properly reconfigured, or when the official Linux port is released. (As of this writing, no native ports are available, but the game's Kickstarter had a stretch goal for one, and that goal was successfully reached.)

Edit: will try to make a guide soon to replace this post, unless the issue is fixed before then