Seems so. Checked the sources and there’s no upload/download. And testing it I can confirm the network tab in the dev tools is silent. Everything work local side. Probably with some additional work this could be converted to a PWA (Progressive Web App) and work completely offline.

The good place

I’m currently playing the Amazon version.

As it’s a code for gog, you can easily use heroic launcher, it works wonderfull.

Go to desktop mode, in the discover app install heroic launcher.

Then open it, go to the store and login to gog. Now you can install any gog games and add it to steam directly from heroic launcher. Then it works perfectly out of the box in game mode.