I forgot about that, I will add it to my earlier reply. You're right, if you want the Linux version you need to purchase the sim here on Itch.
(It's actually intentional - since the Linux version is somewhat experimental and I can't guarantee that it will work for sure I decided not to offer it on Steam. Those who still want it for their Linux machines can get it here. I recommend trying the free version first to see if it works on your particular setup).