I honestly didn't get any hint that this was unfiction until the end with the "it corrupted the disc" at the ending, which is clearly a more than subtle hint that its unfiction,
the way he did the animation, the limited color depth, the fidelity of the models and environments, the sound compression artifacts, even the transitions between screens are spot on to what you'd expect from a 90's myst like at the time.
and on top of all this it doesn't beat you around the head with a stupid, pretentious story. It doesn't have really annoying jumpscares or loud noises. No "haunted game disc" Its actually subtle and leaves it to your imagination as these things should.
I can only wish more unfiction (or ARG as zoomies incorrectly refer to it as) like this comes out, this along with interloper are the gold standard these types of videos should aspire to.