I took a quick look at the github repository (because I'm on macos), and it looks like it is written in javascript (and xml) using "gjs", which exposes the gnome libraries to javascript. So I think it's a gtk/gnome program, but written in javascript instead of C and using a webkit widget for the book itself.

App is written in JS and uses a JS library to render epub in the browser, how else could it work?

epub may contain JS