This is a big deal because Corno worked on Event as a designer/writer and actually contributed to the game's other three endings, as well as a fourth (or fifth?) ending that was cut during development. As he puts it, "we made a game with an ending so secret we didn’t even know it ourselves."
According to Corno, this "secret" fourth ending is actually the result of a bug that the Event team didn't know about until this week, when Corno discovered that the game's Wikipedia page had detailed instructions for how to pull it off.
It's a fun example of how bugs can sometimes be interpreted as features intentionally implemented by the dev team. Corno seems content to see the fourth ending kept in the game, noting that while it contradicts the dev team's rules for how a certain AI character should behave, it makes that character "more human."
As one of the developers, game designers and writers, I know that the game features only three endings. I mean, I wrote them.— Manu (@Yakkafo) July 11, 2017