Why does it have to be called X? We already have x.com -- In fact what is it with this infactuation with calling everything "somethingX" or "X something"
The X name is not meant to represent anything specific. It is simply a placeholder.
When I start a new project, folder, or application before the final identity is decided, I often use a simple name like X, X.app, X-project, etc. until the direction and branding become clearer.
In this case, X-OS is just the temporary name for the operating system project. The final name can change later.