Updates from the devs:
We are working on support for non-16:9 resolutions currently. For time being you’ll be stuck at that 1300ish resolution or playing it windowed, where you can size the window larger than that. Sorry!
No exact ETA on when it will arrive, but yes, the process is starting on our end to get it working.
So it is coming, which means I’ll likely be re-purchasing it!