Here's a recent video from an update last year:
Writing on Steam, the developer put up a rough roadmap of what's to come for the game. In it, they mentioned Linux a few times. What they said, is that they're breaking up into smaller teams to work on different things. One of those teams will be working on the Linux and Mac versions.
Later in the post, they give a bunch of rough dates for various features and here's the important bit for us:
May/June: Linux and Mac builds available.
After that, the full release to leave early access will be sometime in "Q3/Q4" of this year.
On top of that, they're also going to be putting out free content after the final launch. They say this will be sometime between 2-4 months after release and it will come in the form of a "content heavy free DLC"—sounds good to me!
Thanks for the tip Ben!