Security in software is relative. Everything is vulnerable. If someone wants to hack into Google's super-secret data and Google is running Linux or Mac OS X, then the hackers will find exploits in that software. This article is a joke for implying that this switch will in any way make them more secure.
On Windows, they would have to report the vulnerability to Microsoft, and hope that it's patched soon. It probably would be, but that's still an extra step - and they wouldn't be able to locate it with reference to the code. Also, as another person mentioned, china has access to the Windows source code; Google doesn't. This evens the playing field.