A quick peek inside the source code reveals that it's being produced with the use of SugarCube (v2.31.1, specifically). That's just a format or template, really, for a very well-known open-source tool called Twine that's been around for over a decade now and really allows people like the developer of this game to focus on telling a great story without having to futz around with too much of the nuts-and-bolts programming that brings it all to life.
All this stuff is free and cross-platform, and I've linked to their respective homepages if you want to download them and fool around with it yourself. Cheers.
