I think it's a miracle that they managed to finish this game at all. And I'm sure they have burned through the Kickstarter money years ago. It has probably cost them a few times over. That, of course, is not an excuse. It's an explanation.
I know this sucks, so I'd expect the game to work on Linux without issues thanks to Proton. Nightdive is doing a lot of great things for gamers, especially bringing back many classics. I don't think this is done out of malice. Still - a broken promise.