Still, that it was done by one single person, such an immense game, before the Internet. I could pull it off now, (hi google), then, not in a million years.
Also Infocom games were text, much simpler things.
Speaking as someone who's spent a lot of time reading the Zork source code and the Z-machine spec that allowed it to be ported to an huge variety of ancient platforms (http://gunkies.org/wiki/Zork), I have to say, your dismissive attitude is wildly inaccurate.
Both the game and its VM platform were incredibly intricate works of genius.
There is a "surprising" number of one-man shows in the early gaming world.
Also, even though the net didn't exist books did. Not was Another World his first game.
And do not belittle the Infocom games, they had perhaps the best parser of any text adventure. And they were able to cram those games into 8-bit micros.
Also Infocom games were text, much simpler things.