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.
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.
I had a "portable" 286 PC with DOS. And two games. Bubbles Bubbles and a game with two chimps throwing Bananas at each other.
For a game it has to be Outer Wilds...
And please do yourself a favor and play it the one time you're allowed in this lifetime if you've not yet done it, as you can.