The Game Formerly Known as Hidden Nazi Mode is a harmless game about finding bunny rabbits hiding in a Jewish neighborhood. Adorable, right? It could be that the author grew up in a Jewish neighborhood, or the game could be aimed at Jewish kids and attempting to create familiar surroundings, or it could just happen to be one. It's a nice, homey, friendly place that's safe and welcoming.
Until you enter the "special command." (Which has apparently now been removed, as I entered it in the current version and it didn't work.)
Suddenly the game is about a Nazi officer searching for Jewish families. The rabbits are replaced by terrified innocent victims. Both the player character and the game itself become monstrous.
The point, though, is that the horror is hidden -- not the way horror is hidden in a creepypasta game, but deliberately hidden by the creator to pass along an abhorrent message to an intended audience under the nose of the mainstream buyer. And this is absolutely something that a sufficiently 2edgy4u kid could discover.
The author's point is that you never know what's hiding under the surface of the media your kids are consuming. Not unless you can actually examine it, take it apart, and make sure there's nothing under the surface.
I ... don't fully agree with this. It's not that parents shouldn't curate their children's media. That's a matter of carefully walking the line between too restrictive (isolates the child from friends, prevents learning critical thinking during crucial development stages, weakens child's resistance) and too permissive (forces confronting adult ideas at too early a stage, encourages poor conflict resolution, demonstrates unchallenged bad behavior).
What I don't agree with is the notion that seeing publisher-provided source code would accomplish anything at all.
If someone is already perfectly willing to hide rancid, nauseating evil inside a children's game, what on earth makes you think that the source code document will be the same source code as the actual game? If the game is a lie then why would the source code necessarily be the truth!?