Indeed but I would still like an answer from the developer.

Is there a plan to launch Coaster for Windows or Linux? There is no plan for that yet. Although it is technically not difficult to do it, we've decided to focus on the Mac version for the time being.

Edit: I found this on the FAQ section of the website. This leads me to believe that it's likely an Electron app. I don't have anything against Electron and am merely asking out of curiosity.