To add to that, because it's unique: This event is tied to two 'persistent' variables, meaning it isn't tied to a save, it exists outside of saves, you can do it in one save and it will be unlocked for all of them. This means you can just freely jump to it from wherever in the game, finish it and then just close the game without saving, no save of any kind is needed, it's all hidden in the background code.
I feared it might be a classic Selly trap but i think you're just meant to see it when doing a full replay of the game, a sort of easter egg. It doesn't seem to do anything but make it available and then tick it off as having been seen, i'm quite unsure how you could tie it into a Selly trap later on without a good deal of risk for non 'cheating' players.