If you're on Windows or Linux yourself, why not contact them and ask / offer to be a tester? It might be as simple as they haven't gotten around to it, they don't have demand, or they don't have a device to test it themselves.
I'd love to beta-test this on Linux but I'm afraid I won't last for long – I'm just not in the target audience for risograph printing software. I could help setting up a test suite that can then be be run across all platforms, though.
And it seems like they are reluctant to support other platforms:
And they don't need to – Electron tooling can cross-compile just fine nowadays. Of course, to test it you do have to have access to the platform, but even an untested release would be nice.
(That said, I'd probably release it as a webapp instead – unless there's some native code involved, and even that probably could be compiled as Wasm.)
Edit: looked into it, seems they are using a Go executable as the core of the app, which means a naive port (drop app.asar into Electron package for another platform) isn't an option here but it still should be pretty easy to port.
