No.

It is not.

    Object.defineProperty(document, 'visibilityState', { value: "visible", writable: false })

demonstrates trivially that the documentation is clearly wrong.

Maybe it says it's "read-only" because Google wants bad guys to do this sort of thing, since it makes advertisers buy more ads from them.

Or maybe it's an honest mistake that neither Mozilla, nor Google (nor Microsoft or anyone else it seems) has any idea what "read-only" means.