Currently the documentation states "Returns TRUE/FALSE (boolean) if the user agent was referred from another site" however this is not what the function does. The function doesn't check the if the refferer is for the current site and simply returns true if one is set (which it will be for all links within a site). Either the documentation needs updating or this function needs to be updated to do as described.
I'd consider it a bug.
At the very least - everybody can check if the value exists with ! empty($_SERVER['HTTP_REFERER']), so the method should do more than that.