Yeah, I'm surprised about it as well, it looks quite interesting. I wonder if it could be run in Lutris or some other variant of Wine. It looks like a few people have tried, but I don't know how successful they've been...

There is also Co-op on Linux that I just found, that might cut it.

Running a Windows VM just for local co-op isn't bad, but you'd need to download everything again and use windows, wich kinda defeats the point.

You might be able to muck about with routing inputs to a VM or two and basically run two OSs on one computer. If you have an extra video card, monitor, and keyboard, you could actually run two systems on the same computer with Multiseat. That probably requires some more computer resources, but it's basically guaranteed to work. You may need a second steam account for some games though.

The step after that is just getting a second computer and doing it LAN party style.