In "Portal 2", Cave Johnson started out Aperture Science as a shower curtain company.

The statement is true because in the video game "Portal 2," it is revealed through audio recordings by Cave Johnson, the founder of Aperture Science, that the company initially began its operations as a shower curtain manufacturer. These recordings can be found throughout the game's storyline, providing backstory and insight into the fictional history of the company. Thus, Cave Johnson's involvement with Aperture Science indeed started off with a shower curtain business.